GPT2 IndexError: index out of range in functional.py by running run_clm.py when adding any special tokens (even eos and bos only)
Hi all, I need your help as I'm stuck on an issue IndexError trying to finetune GPT2 using **run_clm.py** while adding special tokens. The error is trigger at this line of **functional.py**:
`return torch.embedding(weight, input, padding_idx, scale_grad_by_freq, sparse)` 

**run_clm.py** has been "barely" modified just adding the tokens with tokenizer.add_special_tokens
See below details of the modification, the args used and the error log.

After weeks of preparing datasets, we hope to use your amazing scripts and library for an awesome AI project, I need your help please! 👍 

## Environment info
- `transformers` version: 4.5.0
- Platform: Darwin-20.2.0-x86_64-i386-64bit
- Python version: 3.7.9
- PyTorch version (GPU?): 1.8.1 (False)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: NO
- Using distributed or parallel set-up in script?: NO

Also tried on Windows OS with CUDA 11.1 same transformers version, same Python version, etc = same issue.

### Who can help
@patrickvonplaten, @LysandreJik, @sgugger 

## Information
Model I am using (Bert, XLNet ...): GPT2 Medium

The problem arises when using:
* [X] the official example scripts: (give details below)

The tasks I am working on is:
* [X] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:
1. Run **transformers/examples/language-modeling/run_clm.py** with the following args (see below). You can probably have the exact same issue using any dataset. It doesn't look to be a dataset related issue as the training works without the special tokens added.
2. The file **run_clm.py** has been modified slightly just to include eos token, bos token and additional special tokens (see below). The issue persists as long as I add any of these special token. The only solution seems to be to have no special token at all with this GPT2 fine-tuning code which is unfortunate because I need those for my purpose. :)

**ARGS**

```
python transformers/examples/language-modeling/run_clm.py \
--output_dir "models/output/" \
--model_type "gpt2" \
--model_name_or_path "models/original/" \
--tokenizer_name "gpt2" \
--cache_dir "models/cache/" \
--no_use_fast_tokenizer \
--do_train True \
--train_file "models/datasets/dataset-training-05042021.txt" \
--do_eval True \
--validation_file "models/datasets/dataset-validation-05042021.txt" \
--per_device_train_batch_size 1 \
--per_device_eval_batch_size 1 \
--gradient_accumulation_steps 1 \
--save_steps 500 \
--num_train_epochs 5 \
--learning_rate 5e-5 \
--weight_decay 0 \
--adam_beta1 0.9 \
--adam_beta2 0.999 \
--adam_epsilon 1e-8 \
--max_grad_norm 1.0 \
--no_cuda True \
--seed 123456 \
--fp16 False \
--fp16_opt_level "O1" \
--fp16_backend "auto" \
--fp16_full_eval False \
```

**CODE MODIFICATION**

I added this code on line **308** of **run_clm.py** just before the model.resize_token_embeddings(len(tokenizer)):

```
    special_tokens_dict = {
        'bos_token': '<|startoftext|>',
        'eos_token': '<|endoftext|>',
        'additional_special_tokens': [
             "<A>",
             "<B>",
             "<C>",
             "<D>",
             "<E>",
             "<F>",
             "<G>",
             "<H>"
         ]
    }
    num_added_toks = tokenizer.add_special_tokens(special_tokens_dict)
```

**ISSUE LOGS**

```
04/06/2021 17:48:36 - WARNING - __main__ -   Process rank: -1, device: cpu, n_gpu: 0distributed training: False, 16-bits training: False
04/06/2021 17:48:36 - INFO - __main__ -   Training/evaluation parameters TrainingArguments(output_dir=models/output/, overwrite_output_dir=False, do_train=True, do_eval=True, do_predict=False, evaluation_strategy=IntervalStrategy.NO, prediction_loss_only=False, per_device_train_batch_size=1, per_device_eval_batch_size=1, gradient_accumulation_steps=1, eval_accumulation_steps=None, learning_rate=5e-05, weight_decay=0.0, adam_beta1=0.9, adam_beta2=0.999, adam_epsilon=1e-08, max_grad_norm=1.0, num_train_epochs=5.0, max_steps=-1, lr_scheduler_type=SchedulerType.LINEAR, warmup_ratio=0.0, warmup_steps=0, logging_dir=runs/Apr06_17-48-36_BLABLABLA-MacBook-Air.local, logging_strategy=IntervalStrategy.STEPS, logging_first_step=False, logging_steps=500, save_strategy=IntervalStrategy.STEPS, save_steps=500, save_total_limit=None, no_cuda=True, seed=261184, fp16=False, fp16_opt_level=O1, fp16_backend=auto, fp16_full_eval=False, local_rank=-1, tpu_num_cores=None, tpu_metrics_debug=False, debug=False, dataloader_drop_last=False, eval_steps=500, dataloader_num_workers=0, past_index=-1, run_name=models/output/, disable_tqdm=False, remove_unused_columns=True, label_names=None, load_best_model_at_end=False, metric_for_best_model=None, greater_is_better=None, ignore_data_skip=False, sharded_ddp=[], deepspeed=None, label_smoothing_factor=0.0, adafactor=False, group_by_length=False, length_column_name=length, report_to=['tensorboard'], ddp_find_unused_parameters=None, dataloader_pin_memory=True, skip_memory_metrics=False, _n_gpu=0, mp_parameters=)
04/06/2021 17:48:36 - WARNING - datasets.builder -   Using custom data configuration default-544362d6d13a5db7
04/06/2021 17:48:36 - WARNING - datasets.builder -   Reusing dataset text (/Users/blablabla/.cache/huggingface/datasets/text/default-544362d6d13a5db7/0.0.0/e16f44aa1b321ece1f87b07977cc5d70be93d69b20486d6dacd62e12cf25c9a5)
[INFO|configuration_utils.py:488] 2021-04-06 17:48:36,800 >> loading configuration file models/original/config.json
[INFO|configuration_utils.py:526] 2021-04-06 17:48:36,802 >> Model config GPT2Config {
  "activation_function": "gelu_new",
  "architectures": [
    "GPT2LMHeadModel"
  ],
  "attn_pdrop": 0.1,
  "bos_token_id": 50256,
  "embd_pdrop": 0.1,
  "eos_token_id": 50256,
  "gradient_checkpointing": false,
  "initializer_range": 0.02,
  "layer_norm_epsilon": 1e-05,
  "model_type": "gpt2",
  "n_ctx": 1024,
  "n_embd": 768,
  "n_head": 12,
  "n_inner": null,
  "n_layer": 12,
  "n_positions": 1024,
  "resid_pdrop": 0.1,
  "summary_activation": null,
  "summary_first_dropout": 0.1,
  "summary_proj_to_labels": true,
  "summary_type": "cls_index",
  "summary_use_proj": true,
  "task_specific_params": {
    "text-generation": {
      "do_sample": true,
      "max_length": 50
    }
  },
  "transformers_version": "4.5.0",
  "use_cache": true,
  "vocab_size": 50257
}

[INFO|configuration_utils.py:490] 2021-04-06 17:48:37,245 >> loading configuration file https://huggingface.co/gpt2/resolve/main/config.json from cache at models/cache/fc674cd6907b4c9e933cb42d67662436b89fa9540a1f40d7c919d0109289ad01.7d2e0efa5ca20cef4fb199382111e9d3ad96fd77b849e1d4bed13a66e1336f51
[INFO|configuration_utils.py:526] 2021-04-06 17:48:37,247 >> Model config GPT2Config {
  "activation_function": "gelu_new",
  "architectures": [
    "GPT2LMHeadModel"
  ],
  "attn_pdrop": 0.1,
  "bos_token_id": 50256,
  "embd_pdrop": 0.1,
  "eos_token_id": 50256,
  "gradient_checkpointing": false,
  "initializer_range": 0.02,
  "layer_norm_epsilon": 1e-05,
  "model_type": "gpt2",
  "n_ctx": 1024,
  "n_embd": 768,
  "n_head": 12,
  "n_inner": null,
  "n_layer": 12,
  "n_positions": 1024,
  "resid_pdrop": 0.1,
  "summary_activation": null,
  "summary_first_dropout": 0.1,
  "summary_proj_to_labels": true,
  "summary_type": "cls_index",
  "summary_use_proj": true,
  "task_specific_params": {
    "text-generation": {
      "do_sample": true,
      "max_length": 50
    }
  },
  "transformers_version": "4.5.0",
  "use_cache": true,
  "vocab_size": 50257
}

[INFO|tokenization_utils_base.py:1707] 2021-04-06 17:48:39,085 >> loading file https://huggingface.co/gpt2/resolve/main/vocab.json from cache at models/cache/684fe667923972fb57f6b4dcb61a3c92763ad89882f3da5da9866baf14f2d60f.c7ed1f96aac49e745788faa77ba0a26a392643a50bb388b9c04ff469e555241f
[INFO|tokenization_utils_base.py:1707] 2021-04-06 17:48:39,085 >> loading file https://huggingface.co/gpt2/resolve/main/merges.txt from cache at models/cache/c0c761a63004025aeadd530c4c27b860ec4ecbe8a00531233de21d865a402598.5d12962c5ee615a4c803841266e9c3be9a691a924f72d395d3a6c6c81157788b
[INFO|tokenization_utils_base.py:1707] 2021-04-06 17:48:39,086 >> loading file https://huggingface.co/gpt2/resolve/main/added_tokens.json from cache at None
[INFO|tokenization_utils_base.py:1707] 2021-04-06 17:48:39,086 >> loading file https://huggingface.co/gpt2/resolve/main/special_tokens_map.json from cache at None
[INFO|tokenization_utils_base.py:1707] 2021-04-06 17:48:39,086 >> loading file https://huggingface.co/gpt2/resolve/main/tokenizer_config.json from cache at None
[INFO|tokenization_utils_base.py:1707] 2021-04-06 17:48:39,086 >> loading file https://huggingface.co/gpt2/resolve/main/tokenizer.json from cache at models/cache/16a2f78023c8dc511294f0c97b5e10fde3ef9889ad6d11ffaa2a00714e73926e.cf2d0ecb83b6df91b3dbb53f1d1e4c311578bfd3aa0e04934215a49bf9898df0
[INFO|modeling_utils.py:1050] 2021-04-06 17:48:39,223 >> loading weights file models/original/pytorch_model.bin
[INFO|modeling_utils.py:1168] 2021-04-06 17:48:45,948 >> All model checkpoint weights were used when initializing GPT2LMHeadModel.

[INFO|modeling_utils.py:1177] 2021-04-06 17:48:45,949 >> All the weights of GPT2LMHeadModel were initialized from the model checkpoint at models/original/.
If your task is similar to the task the model of the checkpoint was trained on, you can already use GPT2LMHeadModel for predictions without further training.
[INFO|tokenization_utils_base.py:873] 2021-04-06 17:48:45,949 >> Assigning <|startoftext|> to the bos_token key of the tokenizer
[INFO|tokenization_utils.py:207] 2021-04-06 17:48:45,950 >> Adding <|startoftext|> to the vocabulary
[INFO|tokenization_utils_base.py:873] 2021-04-06 17:48:45,950 >> Assigning <|endoftext|> to the eos_token key of the tokenizer
[INFO|tokenization_utils_base.py:873] 2021-04-06 17:48:45,950 >> Assigning ['<A>', '<B>', '<C>', '<D>', '<E>', '<F>', '<G>', '<H>'] to the additional_special_tokens key of the tokenizer
[INFO|tokenization_utils.py:207] 2021-04-06 17:48:45,950 >> Adding <A> to the vocabulary
[INFO|tokenization_utils.py:207] 2021-04-06 17:48:45,950 >> Adding <B> to the vocabulary
[INFO|tokenization_utils.py:207] 2021-04-06 17:48:45,950 >> Adding <C> to the vocabulary
[INFO|tokenization_utils.py:207] 2021-04-06 17:48:45,950 >> Adding <D> to the vocabulary
[INFO|tokenization_utils.py:207] 2021-04-06 17:48:45,950 >> Adding <E> to the vocabulary
[INFO|tokenization_utils.py:207] 2021-04-06 17:48:45,950 >> Adding <F> to the vocabulary
[INFO|tokenization_utils.py:207] 2021-04-06 17:48:45,950 >> Adding <G> to the vocabulary
[INFO|tokenization_utils.py:207] 2021-04-06 17:48:45,950 >> Adding <H> to the vocabulary
100%|██████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 199/199 [01:15<00:00,  2.62ba/s]
100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 10/10 [00:03<00:00,  2.69ba/s]
100%|██████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 199/199 [01:02<00:00,  3.17ba/s]
100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 10/10 [00:02<00:00,  3.39ba/s]
[INFO|trainer.py:921] 2021-04-06 17:51:21,859 >> Loading model from models/original/).
[INFO|configuration_utils.py:488] 2021-04-06 17:51:21,924 >> loading configuration file models/original/config.json
[INFO|configuration_utils.py:526] 2021-04-06 17:51:21,931 >> Model config GPT2Config {
  "activation_function": "gelu_new",
  "architectures": [
    "GPT2LMHeadModel"
  ],
  "attn_pdrop": 0.1,
  "bos_token_id": 50256,
  "embd_pdrop": 0.1,
  "eos_token_id": 50256,
  "gradient_checkpointing": false,
  "initializer_range": 0.02,
  "layer_norm_epsilon": 1e-05,
  "model_type": "gpt2",
  "n_ctx": 1024,
  "n_embd": 768,
  "n_head": 12,
  "n_inner": null,
  "n_layer": 12,
  "n_positions": 1024,
  "resid_pdrop": 0.1,
  "summary_activation": null,
  "summary_first_dropout": 0.1,
  "summary_proj_to_labels": true,
  "summary_type": "cls_index",
  "summary_use_proj": true,
  "task_specific_params": {
    "text-generation": {
      "do_sample": true,
      "max_length": 50
    }
  },
  "transformers_version": "4.5.0",
  "use_cache": true,
  "vocab_size": 50257
}

[INFO|modeling_utils.py:1050] 2021-04-06 17:51:21,950 >> loading weights file models/original/pytorch_model.bin
[INFO|modeling_utils.py:1168] 2021-04-06 17:51:31,409 >> All model checkpoint weights were used when initializing GPT2LMHeadModel.

[INFO|modeling_utils.py:1177] 2021-04-06 17:51:31,409 >> All the weights of GPT2LMHeadModel were initialized from the model checkpoint at models/original/.
If your task is similar to the task the model of the checkpoint was trained on, you can already use GPT2LMHeadModel for predictions without further training.
[INFO|trainer.py:1013] 2021-04-06 17:51:31,478 >> ***** Running training *****
[INFO|trainer.py:1014] 2021-04-06 17:51:31,483 >>   Num examples = 8199
[INFO|trainer.py:1015] 2021-04-06 17:51:31,489 >>   Num Epochs = 5
[INFO|trainer.py:1016] 2021-04-06 17:51:31,489 >>   Instantaneous batch size per device = 1
[INFO|trainer.py:1017] 2021-04-06 17:51:31,489 >>   Total train batch size (w. parallel, distributed & accumulation) = 1
[INFO|trainer.py:1018] 2021-04-06 17:51:31,489 >>   Gradient Accumulation steps = 1
[INFO|trainer.py:1019] 2021-04-06 17:51:31,489 >>   Total optimization steps = 40995
  0%|                                                                                                                                                                              | 0/40995 [00:00<?, ?it/s]Traceback (most recent call last):
  File "transformers/examples/language-modeling/run_clm.py", line 459, in <module>
    main()
  File "transformers/examples/language-modeling/run_clm.py", line 424, in main
    train_result = trainer.train(resume_from_checkpoint=checkpoint)
  File "/Users/blablabla/Developer/Training/env/lib/python3.7/site-packages/transformers/trainer.py", line 1120, in train
    tr_loss += self.training_step(model, inputs)
  File "/Users/blablabla/Developer/Training/env/lib/python3.7/site-packages/transformers/trainer.py", line 1524, in training_step
    loss = self.compute_loss(model, inputs)
  File "/Users/blablabla/Developer/Training/env/lib/python3.7/site-packages/transformers/trainer.py", line 1556, in compute_loss
    outputs = model(**inputs)
  File "/Users/blablabla/Developer/Training/env/lib/python3.7/site-packages/torch/nn/modules/module.py", line 889, in _call_impl
    result = self.forward(*input, **kwargs)
  File "/Users/blablabla/Developer/Training/env/lib/python3.7/site-packages/transformers/models/gpt2/modeling_gpt2.py", line 917, in forward
    return_dict=return_dict,
  File "/Users/blablabla/Developer/Training/env/lib/python3.7/site-packages/torch/nn/modules/module.py", line 889, in _call_impl
    result = self.forward(*input, **kwargs)
  File "/Users/blablabla/Developer/Training/env/lib/python3.7/site-packages/transformers/models/gpt2/modeling_gpt2.py", line 694, in forward
    inputs_embeds = self.wte(input_ids)
  File "/Users/blablabla/Developer/Training/env/lib/python3.7/site-packages/torch/nn/modules/module.py", line 889, in _call_impl
    result = self.forward(*input, **kwargs)
  File "/Users/blablabla/Developer/Training/env/lib/python3.7/site-packages/torch/nn/modules/sparse.py", line 158, in forward
    self.norm_type, self.scale_grad_by_freq, self.sparse)
  File "/Users/blablabla/Developer/Training/env/lib/python3.7/site-packages/torch/nn/functional.py", line 1921, in embedding
    return torch.embedding(weight, input, padding_idx, scale_grad_by_freq, sparse)
IndexError: index out of range in self
  0%|                                                                                                                                                                              | 0/40995 [00:00<?, ?it/s]
```
Hi @MoonshotQuest, this is because you haven't resized your token embedding matrix after adding new tokens to your vocabulary. The tokenizer can therefore generate new tokens, but the model doesn't know how to handle them. The `add_tokens` method has a [note for that](https://huggingface.co/transformers/internal/tokenization_utils.html?highlight=add_special#transformers.tokenization_utils_base.SpecialTokensMixin.add_tokens)

It was unfortunately forgotten for the `add_special_tokens` method and only put in the example, so I'm updating it.<|||||>Hi @LysandreJik - Thanks so much for looking into it!
I did check your note and I think I'm already resizing the token embedding matrix as I add my code on line 308.
Line 309 (unchanged) is already: **model.resize_token_embeddings(len(tokenizer))**
https://github.com/huggingface/transformers/blob/master/examples/language-modeling/run_clm.py

That gives the following:
```
    special_tokens_dict = {
        'bos_token': '<|startoftext|>',
        'eos_token': '<|endoftext|>',
        'additional_special_tokens': [
             "<A>",
             "<B>",
             "<C>",
             "<D>",
             "<E>",
             "<F>",
             "<G>",
             "<H>"
         ]
    }
    num_added_toks = tokenizer.add_special_tokens(special_tokens_dict)
    model.resize_token_embeddings(len(tokenizer))
```

Is there another matrix to resize? 
<|||||>I am unable to reproduce your issue. In my side adding the code you mentioned to the script runs perfectly with
```
python examples/language-modeling/run_clm.py \
  --model_type gpt2 \
  --tokenizer_name gpt2 \
  --dataset_name wikitext \
  --dataset_config_name wikitext-2-raw-v1 \
  --do_train \
  --do_eval \
  --output_dir ~/tmp/tst-clm \
  --block_size 128 \
  --max_train_samples 100 \
  --overwrite_output_dir \
  --no_use_fast_tokenizer
``` 

I am using wikitext2 since I don't have access to your dataset. Note that your command contains statements that are a bit contradictory:
- you are mixing a `model_name_or_path` (fine-tuning an existing model) with `model_type` (training from scratch).
- you are mixiing `no_cuda` (so training on CPU) with FP16 options, which are not supported on CPU.<|||||>Hi @sgugger thanks for trying to reproduce!

I removed the GPU related args and isolated the issue to the use of my own folder & pre-trained model:
`--model_type gpt2` # Works perfectly
`--model_name_or_path "models/original/"`  # Doesn't work and throw the IndexError

I believe the issue is the model files I'm using in my folder `models/original/` as the pre-trained GPT2 Medium. They seem to be different than the ones downloaded and cached when using the `--model_type gpt2` argument. I only have 2 files in the folder the .bin and the .config. I would like to keep these files in a folder offline as a precaution. 
I pulled the files from these URL. Is there a different bin file used for fine-tuning vs a file for inferences only?
https://huggingface.co/gpt2-medium/resolve/main/config.json
https://s3.amazonaws.com/models.huggingface.co/bert/gpt2-medium-pytorch_model.bin

I see in some other part of transformers code the following, which may suggest that different pre-trained models .bin and .config files are used for different purpose. Maybe I'm completely wrong! 😂
Thanks for your guidance on this.
```
    _import_structure["models.gpt2"].extend(
        [
            "GPT2_PRETRAINED_MODEL_ARCHIVE_LIST",
            "GPT2DoubleHeadsModel",
            "GPT2ForSequenceClassification",
            "GPT2LMHeadModel",
            "GPT2Model",
            "GPT2PreTrainedModel",
            "load_tf_weights_in_gpt2",
        ]
    )
```<|||||>Ah, this is because your checkpoint should have the resized weights: it's resized inside the script but since it's a local folder, it's also passed as a checkpoint to the Trainer later in the script, which then reloads the model from that folder without the `model.resize_token_embeddings(len(tokenizer))` this time. So you have two solutions:
- either load your model, apply `model.resize_token_embeddings(len(tokenizer))` then resave it.
- or remove the line that interprets the folder as a checkpoint [here](https://github.com/huggingface/transformers/blob/c6d664849bdc580cf813b2d3a555a9b33d31b33d/examples/language-modeling/run_clm.py#L414)<|||||>Great thank you so much! @sgugger @LysandreJik 
That makes sense now, I removed the line and it works perfectly. 👍

I will let you know when we get closer to a launch date for our AI based game. It's going to be awesome!
Sorry to troll this thread but does Huggingface has a place to showcase apps made using your incredible libraries? 😊<|||||>We have a [community page](https://huggingface.co/transformers/community.html) in the documentation, otherwise we'll be happy to help you share on social media!<|||||>Awesome!! Take care 🤗