Small change to Wav2Vec2 model to support Tensor-Parallelism with DeepSpeed
# What does this PR do?

This PR adds a minor modification to BartAttention and its copies to support tensor-parallelism with DeepSpeed. This relates to this [PR](https://github.com/microsoft/DeepSpeed/pull/1512) on DeepSpeed side.

Please see the added comments in the code that explain the change.

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## Before submitting
- [ ] 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.
- [ ] 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?

@stas00 

> Could you also add a test to make sure the feature works (we might not be able to run it on our 2 GPUs machine, but a 4 GPUs one is coming).

We will have a full battery of tests for Deepspeed Inference. I will take care of this, Sylvain. The plan is to have a model zoo style test - identical to Deepspeed ZeRO tests, so to cover as many models as possible. (there will be also Deepspeed ZeRO Inference tests https://github.com/huggingface/transformers/pull/14253, which is different from Deepspeed Inference)

We didn't feel a test was needed for this particular PR since it doesn't change anything for a normal application.<|||||>You now need to run `make style` on your branch to fix the code quality issue :-)<|||||>Thanks @sgugger and @stas00 