Bug in Marian model (or tokenizer) in transformers==4.18.0
## Environment info
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- `transformers` version: 4.18.0
- Platform: Google Colab / Linux & Conda
- Python version: 3.7.13
- PyTorch version (GPU?): 1.10.0
- Using GPU in script?: Yes
- Using distributed or parallel set-up in script?: No

### Who can help
@patil-suraj

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Models:

- ALBERT, BERT, XLM, DeBERTa, DeBERTa-v2, ELECTRA, MobileBert, SqueezeBert: @LysandreJik
- T5, Pegasus, EncoderDecoder: @patrickvonplaten
- Blenderbot, MBART, BART, Marian, Pegasus: @patil-suraj
- Reformer, TransfoXL, XLNet, FNet: @patrickvonplaten
- Longformer, BigBird: @ydshieh
- FSMT: @stas00
- Funnel: @sgugger
- GPT-2, GPT: @patil-suraj, @patrickvonplaten, @LysandreJik
- RAG, DPR: @patrickvonplaten, @lhoestq
- TensorFlow: @Rocketknight1
- JAX/Flax: @patil-suraj
- TAPAS, LayoutLM, LayoutLMv2, LUKE, ViT, BEiT, DEiT, DETR, CANINE: @NielsRogge
- GPT-Neo, GPT-J, CLIP: @patil-suraj
- Wav2Vec2, HuBERT, SpeechEncoderDecoder, UniSpeech, UniSpeechSAT, SEW, SEW-D, Speech2Text: @patrickvonplaten, @anton-l

If the model isn't in the list, ping @LysandreJik who will redirect you to the correct contributor.

Library:

- Benchmarks: @patrickvonplaten
- Deepspeed: @stas00
- Ray/raytune: @richardliaw, @amogkam
- Text generation: @patrickvonplaten @narsil
- Tokenizers: @SaulLu
- Trainer: @sgugger
- Pipelines: @Narsil
- Speech: @patrickvonplaten, @anton-l
- Vision: @NielsRogge, @sgugger

Documentation: @sgugger

Model hub:

- for issues with a model, report at https://discuss.huggingface.co/ and tag the model's creator.

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj

For research projetcs, please ping the contributor directly. For example, on the following projects:

- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

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## Information

Model I am using (Bert, XLNet ...): Marian

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

The tasks I am working on is:
* [x] an official GLUE/SQUaD task: Oscar & ALT - Standard MT task
* [ ] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. Extend the tokenizer using a target one
2. Add tokens
3. Run forward with model in training mode.
4. script and error reported here: https://colab.research.google.com/drive/1utS-L1iO1paiwKKPNqVHW5ARvprfRgG2?usp=sharing

**Traceback below:**
```
[/usr/local/lib/python3.7/dist-packages/transformers/models/marian/modeling_marian.py](https://localhost:8080/#) in forward(self, input_ids, attention_mask, decoder_input_ids, decoder_attention_mask, head_mask, decoder_head_mask, cross_attn_head_mask, encoder_outputs, past_key_values, inputs_embeds, decoder_inputs_embeds, labels, use_cache, output_attentions, output_hidden_states, return_dict)
   1452         if labels is not None:
   1453             loss_fct = CrossEntropyLoss()
-> 1454             masked_lm_loss = loss_fct(lm_logits.view(-1, self.target_vocab_size), labels.view(-1))
   1455 
   1456         if not return_dict:

RuntimeError: shape '[-1, 65001]' is invalid for input of size 8320768
```

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## Expected behavior

Standard Marian training output. No issue with `transformers==4.17.0`

<!-- A clear and concise description of what you would expect to happen. -->

Good catch! Fix is here #16700<|||||>Thank you!