add support to DistilBertLMHeadModel
# What does this PR do?

Fixes #14737 
Goal: Add support to DistilBertLMHeadModel
Changes: 

1. Modified [modeling_distilbert.py](https://github.com/huggingface/transformers/blob/master/src/transformers/models/distilbert/modeling_distilbert.py): Changed `MultiHeadSelfAttention`, `Transformer`, `TransformerBlock` to model cross-attention and accept arguments `encoder_hidden_states` & `encoder_attention_mask `; and added the new class `DistilBertLMHeadModel`;
2. Expose the new class and register it everywhere needed.

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?

## Who can review?
@VictorSanh @thomwolf @patil-suraj 
Thanks for the PR! There's a similar PR here #11085, which is almost complete, so let's wait for the author to see if he's still interested to finish it :) <|||||>> Thanks for the PR! There's a similar PR here #11085, which is almost complete, so let's wait for the author to see if he's still interested to finish it :)

Thanks for your attention! OK, let me also have a look at what is going on there and anything I can help:).<|||||>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.