[Possible Bug] Getting IndexError: list index out of range when fine-tuning custom LM model
## Environment info

    `transformers` version: 4.3.3
    Platform: Linux-4.19.112+-x86_64-with-Ubuntu-18.04-bionic
    Python version: 3.7.10
    PyTorch version (GPU?): 1.7.1+cu101 (False)
    Tensorflow version (GPU?): 2.4.1 (False)
    Using GPU in script?: True/False
    Using distributed or parallel set-up in script?: False


### Who can help
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- tokenizers: @LysandreJik
- trainer: @sgugger
## Information

Model I am using (Bert, XLNet ...): `LongFormer`

The problem arises when using:
* [x] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [x] my own task or dataset: (give details below)

## To reproduce

Hi, I am trying to train an LM model on a custom dataset (which is simply text over multiple lines). My choice was the Longformer, and I am using the exact same code provided officially with just a few modifications.

When I fine-tune it on a custom dataset, I am getting this error:-
```py
---------------------------------------------------------------------------

IndexError                                Traceback (most recent call last)

<ipython-input-54-2f2d9c2c00fc> in <module>()
     45     )
     46 
---> 47 train_results = trainer.train()

6 frames

/usr/local/lib/python3.7/dist-packages/transformers/trainer.py in train(self, resume_from_checkpoint, trial, **kwargs)
   1032             self.control = self.callback_handler.on_epoch_begin(self.args, self.state, self.control)
   1033 
-> 1034             for step, inputs in enumerate(epoch_iterator):
   1035 
   1036                 # Skip past any already trained steps if resuming training

/usr/local/lib/python3.7/dist-packages/torch/utils/data/dataloader.py in __next__(self)
    515             if self._sampler_iter is None:
    516                 self._reset()
--> 517             data = self._next_data()
    518             self._num_yielded += 1
    519             if self._dataset_kind == _DatasetKind.Iterable and \

/usr/local/lib/python3.7/dist-packages/torch/utils/data/dataloader.py in _next_data(self)
    555     def _next_data(self):
    556         index = self._next_index()  # may raise StopIteration
--> 557         data = self._dataset_fetcher.fetch(index)  # may raise StopIteration
    558         if self._pin_memory:
    559             data = _utils.pin_memory.pin_memory(data)

/usr/local/lib/python3.7/dist-packages/torch/utils/data/_utils/fetch.py in fetch(self, possibly_batched_index)
     42     def fetch(self, possibly_batched_index):
     43         if self.auto_collation:
---> 44             data = [self.dataset[idx] for idx in possibly_batched_index]
     45         else:
     46             data = self.dataset[possibly_batched_index]

/usr/local/lib/python3.7/dist-packages/torch/utils/data/_utils/fetch.py in <listcomp>(.0)
     42     def fetch(self, possibly_batched_index):
     43         if self.auto_collation:
---> 44             data = [self.dataset[idx] for idx in possibly_batched_index]
     45         else:
     46             data = self.dataset[possibly_batched_index]

<ipython-input-53-5e4959dcf50c> in __getitem__(self, idx)
      7 
      8     def __getitem__(self, idx):
----> 9         item = {key: torch.tensor(val[idx]) for key, val in self.encodings.items()}
     10         item['labels'] = torch.tensor(self.labels[idx])
     11         return item

<ipython-input-53-5e4959dcf50c> in <dictcomp>(.0)
      7 
      8     def __getitem__(self, idx):
----> 9         item = {key: torch.tensor(val[idx]) for key, val in self.encodings.items()}
     10         item['labels'] = torch.tensor(self.labels[idx])
     11         return item

IndexError: list index out of range
```
Most probably it is a tokenization problem, but I can't seem to locate it. 
I ensured that the tokenizer in the **LM** does accept an appropriate length (even if it is quite bigger than I want):     

`tokenizer = LongformerTokenizerFast.from_pretrained("./ny_model", max_len=3500)`

For fine-tuning, I ensured that it would truncate&pad, though none of my data samples are long enough to truncate:

```py
train_encodings = tokenizer(list(train_text), truncation=True, padding=True, max_length=3500)
val_encodings = .....
```
Finally, I tried with some dummy data with _fixed length_ like this:
```py
train_text = ['a', 'b']
val_text = ['c', 'd']
```
Which rules out most tokenization errors. 
I am fine-tuning in accordance to official scripts - something I have done before. the LM looks good to me and tokenizes individually as well, so I have no reason to suspect it. 

I am attaching my **LM** code:-
```py
!pip install -q git+https://github.com/huggingface/transformers
!pip list | grep -E 'transformers|tokenizers'

%%time 
from pathlib import Path

from tokenizers import ByteLevelBPETokenizer

# Initialize a tokenizer
tokenizer = ByteLevelBPETokenizer()

# Customize training
tokenizer.train(files='./NYA.txt', vocab_size=52_000, min_frequency=2, special_tokens=[
    "<s>",
    "<pad>",
    "</s>",
    "<unk>",
    "<mask>",
])

!mkdir ny_model
tokenizer.save_model("ny_model")

from transformers import LongformerConfig

config = LongformerConfig(
    vocab_size=52_000,
    max_position_embeddings=514,
    num_attention_heads=2,
    num_hidden_layers=1,
    type_vocab_size=1,
)

from transformers import LongformerTokenizerFast

tokenizer = LongformerTokenizerFast.from_pretrained("./ny_model", max_len=3500)

from transformers import LongformerForMaskedLM

model = LongformerForMaskedLM(config=config)

%%time
from transformers import LineByLineTextDataset

dataset = LineByLineTextDataset(
    tokenizer=tokenizer,
    file_path="./NYA.txt",
    block_size=128,
)
from transformers import DataCollatorForLanguageModeling

data_collator = DataCollatorForLanguageModeling(
    tokenizer=tokenizer, mlm=True, mlm_probability=0.15
)
from transformers import Trainer, TrainingArguments

training_args = TrainingArguments(
    output_dir="./results",
    overwrite_output_dir=True,
    num_train_epochs=2,
    per_device_train_batch_size=64,
    save_steps=10_000,
    save_total_limit=2,
    prediction_loss_only=True,
    learning_rate=1e-5,
    logging_steps=50,
    fp16=True
)

trainer = Trainer(
    model=model,
    args=training_args,
    train_dataset=dataset,
    data_collator=data_collator
)
trainer.train()
```
and as said again, the fine-tuning part is again just like the official scripts, save the tokenizer arguments and some simple training args.

I believe that this code with a simple dummy dataset could reproduce the bug. I can provide further help on the gist if someone can create one for full reproducibility. If there is some idiotic mistake I have made, please don't hesitate to point that out.

> Any Ideas what the problem might be?

Cheers
Save problem with #10947 but it is still unsolved. Can anyone of HuggingFace's authors please help us out on this issue?
<|||||>The issue you are mentioning concerns TensorFlow models which you are not using so there is no link. In your case, tehre is a problem with your dataset (this is where the error arises from the stack trace you copied) but you are using a class that is not maintained anymore, so you should switch to preprocessing with the datasets library like in [this notebook](https://github.com/huggingface/notebooks/blob/master/examples/language_modeling.ipynb).<|||||>Thanks for the heads-up @sgugger! As a personal favour, can you please convey via the internal channels to update their colab notebooks? [this notebook](https://colab.research.google.com/github/huggingface/blog/blob/master/notebooks/01_how_to_train.ipynb) is the first search result for training LM from scratch and it was updated a year ago - which makes it difficult for anyone to judge what support for classes has been dropped.<|||||>Yes, creating an updated version of this notebook is on our roadmap.<|||||>So I tried doing the exact same way - the LM does converge but when fine-tuning, I am wracked with weird errors - https://stackoverflow.com/questions/67004233/typeerror-zeros-like-argument-input-when-fine-tuning-on-mlm if anyone has any idea or suggestions, can they please convey it to me? I am completely out of ideas.<|||||>You should use the [forums](https://discuss.huggingface.co/) instead of stack overflow, there will be more people to answer your questions there.<|||||>@sgugger I have posted this in the [forum](https://discuss.huggingface.co/t/error-when-fine-tuning-pretrained-masked-language-model/5386/6) but there is a discrepancy - using `datasets` API again for fine-tuning leads to the same error. I tried with quite different variants of tokenization functions, but this error persists. 

Is there another new way to fine-tune models?<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.