Fix cross-attention head mask for Torch encoder-decoder models
1. This PR fixes head masking for the cross-attention module in the following models:

- BART,
- Blenderbot,
- Blenderbot_small,
- FSMT,
- LED,
- M2M_100,
- Marian,
- MBart,
- Pegasus.
- T5

2. This PR also contains slight changes in docstrings so that it will be clear that `head_mask` is related to the config of an encoder and the shape of `decoder_head_mask` and `cross_head_mask` depends on the config of a decoder.
 
3. This PR enables `test_headmasking` for M2M_100 model.

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**Reviewers:** @patrickvonplaten @patil-suraj 

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Fixes: #10540 
Hi @patrickvonplaten & @patil-suraj, my PR does not pass one test, however, I am not able to reproduce this error on my local (I can't even find the file at `src/transformers/models/new_enc_dec/modeling_new_enc_dec.py` in the repo, which is the one where should be a problem with a copy inconsistency)<|||||>Hi @stancld thank you for your work! The issue is because you have updated all model files (thank you!!), but you haven't updated the template. The template is used when adding a new model, it's available [here](https://github.com/huggingface/transformers/tree/master/templates/adding_a_new_model).

For example, these lines should probably be updated to include the `cross_head_mask`:

https://github.com/huggingface/transformers/blob/44f64132a5f50726f9de4467ed745421c3b11ab3/templates/adding_a_new_model/cookiecutter-template-%7B%7Bcookiecutter.modelname%7D%7D/modeling_%7B%7Bcookiecutter.lowercase_modelname%7D%7D.py#L2070-L2085<|||||>@LysandreJik - Thank you very much for the clarification :)<|||||>Wonderful, the tests pass! Thanks for handling the templates.<|||||>@patil-suraj Thank you for your review and the suggestions. I agree with the change in variable naming. The new one is more accurate. I will change it everywhere accordingly :) <|||||>Hi @patrickvonplaten, I've just rebased this branch to the current master to keep this PR up to date. Could you please review this one? Thanks a lot! :) <|||||>Hey @stancld, Patrick is off for a couple of weeks but will take a look at this as soon as he's back :)<|||||>The PR looks good to me :-) 

Think we just need to fix the docstring. @stancld - let me know if you need help regarding the docstring<|||||>@patrickvonplaten The docstring should be fixed now. I forgot a line from a conflict there.. :)