TFLongformerForMaskedMLM example throws ValueError "shapes are incompatible"
An official example of the `TFLongFormerX` page does not work. 

## Environment info
- `transformers` version: 2.4.1
- Platform: ubuntu 20.04
- Python version: python3.8
- PyTorch version (GPU?): N/A
- Tensorflow version (GPU?): 2.4.1
- Using GPU in script?: no
- Using distributed or parallel set-up in script?: no

### Who can help
@patrickvonplaten (Longformer)
@Rocketknight1 (tensorflow)
@sgugger (maintained examples )

## Information
Model I am using: Longformer

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

## To reproduce

Steps to reproduce the behavior:

1. `docker run -it --rm python:3.8 bash` (no gpus attached)
2. `python3 -m pip install pip --upgrade`
3. `python3 -m pip install transformers tensorflow`
4. `python3` -> launch interactive shell
5.  run following lines: 
```
from transformers import LongformerTokenizer, TFLongformerForMaskedLM
import tensorflow as tf
tokenizer = LongformerTokenizer.from_pretrained('allenai/longformer-base-4096')
model = TFLongformerForMaskedLM.from_pretrained('allenai/longformer-base-4096')
inputs = tokenizer("The capital of France is [MASK].", return_tensors="tf")
inputs["labels"] = tokenizer("The capital of France is Paris.", return_tensors="tf")["input_ids"]
outputs = model(inputs)
# loss = outputs.loss
# logits = outputs.logits
```
This throws following error:

```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.8/site-packages/tensorflow/python/keras/engine/base_layer.py", line 1012, in __call__
    outputs = call_fn(inputs, *args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/transformers/models/longformer/modeling_tf_longformer.py", line 2140, in call
    loss = None if inputs["labels"] is None else self.compute_loss(inputs["labels"], prediction_scores)
  File "/usr/local/lib/python3.8/site-packages/transformers/modeling_tf_utils.py", line 158, in compute_loss
    reduced_logits = tf.boolean_mask(tf.reshape(logits, (-1, shape_list(logits)[2])), active_loss)
  File "/usr/local/lib/python3.8/site-packages/tensorflow/python/util/dispatch.py", line 201, in wrapper
    return target(*args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/tensorflow/python/ops/array_ops.py", line 1831, in boolean_mask_v2
    return boolean_mask(tensor, mask, name, axis)
  File "/usr/local/lib/python3.8/site-packages/tensorflow/python/util/dispatch.py", line 201, in wrapper
    return target(*args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/tensorflow/python/ops/array_ops.py", line 1751, in boolean_mask
    shape_tensor[axis:axis + ndims_mask].assert_is_compatible_with(shape_mask)
  File "/usr/local/lib/python3.8/site-packages/tensorflow/python/framework/tensor_shape.py", line 1134, in assert_is_compatible_with
    raise ValueError("Shapes %s and %s are incompatible" % (self, other))
ValueError: Shapes (11,) and (9,) are incompatible
```

Hi! The model is working fine here, but the problem is that "[MASK]" and "Paris" are being tokenized as different numbers of tokens, which is where your shape error is coming from. Can you link me to the exact script you got this example from?<|||||>It's under this headline, here's the permalink: https://huggingface.co/transformers/model_doc/longformer.html#tflongformerformaskedlm<|||||>ah so it's probably just updating `inputs["labels"] = tokenizer("The capital of France is Paris.", return_tensors="tf")["input_ids"]
` to `inputs["labels"] = tokenizer("The capital of [MASK] is Paris.", return_tensors="tf")["input_ids"]`, no?<|||||>I checked and you're absolutely right, the example as written does not work. I did some digging and the problem is that the mask sequence for this model is actually '\<mask\>' and not '[MASK]'. Therefore, 'Paris' actually does get correctly tokenized as one token but '[MASK]' does not get recognized as a special character and is 'spelled out' with three word-piece tokens instead. (You can see what splits the tokenizer chose by using `tokenizer.convert_ids_to_tokens()` on the tokenized inputs).

The example should work if you replace '[MASK]' with '\<mask\>'. Can you try that and let me know? If it works, we can make a PR to fix this example!<|||||>So now the following example:

```from transformers import LongformerTokenizer, TFLongformerForMaskedLM
import tensorflow as tf
tokenizer = LongformerTokenizer.from_pretrained('allenai/longformer-base-4096')
model = TFLongformerForMaskedLM.from_pretrained('allenai/longformer-base-4096')
inputs = tokenizer("The capital of France is <mask>.", return_tensors="tf")
inputs["labels"] = tokenizer("The capital of France is Paris.", return_tensors="tf")["input_ids"]
outputs = model(inputs)
loss = outputs.loss
logits = outputs.logits
preds = tf.argmax(logits, axis=2)
predicted_tokens = tokenizer.convert_ids_to_tokens(tf.squeeze(preds))
print("predicted_tokens: ", predicted_tokens)
```

yields:

`['<s>', 'The', 'Ġcapital', 'Ġof', 'ĠFrance', 'Ġis', 'ĠParis', '.', '</s>']`

So at least we're doing something right, but there's still this weird `Ġ` character on every non-first token.<|||||>Ah, yes! The Ġ character is used to indicate word breaks. If you want to see the pure string output without it, try using the `decode()` method instead of `convert_ids_to_tokens()`.

Other than that, though, your example looks good! I talked with people on the team and we can't use it directly, annoyingly - the examples are all built from the same template, so we can't easily change just one. Still, we can pass some arguments to make sure our example works for Longformer in future.

The relevant bit is [here](https://github.com/huggingface/transformers/blob/master/src/transformers/models/longformer/modeling_tf_longformer.py#L2080). If you'd like to try it yourself, you can submit a PR to add the argument `mask='<mask>'` to the `add_code_sample_docstrings` decorator. If that sounds like a lot of work, just let me know and I'll make the PR and credit you for spotting it!<|||||>@Rocketknight1 I added a PR (https://github.com/huggingface/transformers/pull/11559)<|||||>Closing this because we have the PR now!