fix pytorch division warning by using suggested torch.div rounding_mode
# What does this PR do?
This PR removes a warning that is repeatedly thrown with the latest releases of transformers and pytorch.

example:

```shell
lib/python3.7/site-packages/transformers/models/hubert/modeling_hubert.py:803: UserWarning: __floordiv__ is deprecated, and its behavior will change in a future version of pytorch. It currently rounds toward 0 (like the 'trunc' function NOT 'floor'). This results in incorrect rounding for negative values. To keep the current behavior, use torch.div(a, b, rounding_mode='trunc'), or for actual floor division, use torch.div(a, b, rounding_mode='floor').
```

The fact that this occurs in multiple places suggests there's an opportunity for a shared function. Instead of a larger refactor, this PR touches a few specific places using `//` to avoid causing other side effects.

## 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?
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No
- [ ] 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).

Docs should be unaffected
- [ ] Did you write any new necessary tests?

The test coverage should be unchanged.
Note that this argument of `torch.div` was only introduced in PyTorch 1.8.0, so this fix is not compatible with all the versions of torch we support (1.4.0 and onward). I think we will need to write a function that uses the old syntax (`//`) for older versions and only switches to the newer syntax in more recent versions, then use that function.<|||||>> Note that this argument of `torch.div` was only introduced in PyTorch 1.8.0, so this fix is not compatible with all the versions of torch we support (1.4.0 and onward). I think we will need to write a function that uses the old syntax (`//`) for older versions and only switches to the newer syntax in more recent versions, then use that function.

Agree! Let's put it in `src/transformers/file_utils.py` no? @mgoldey - would be interested in adapting the PR to include such a function? The function could look very similar to this syntax: https://github.com/huggingface/transformers/blob/bc8a9f415b15d0c8e2f01c6ad79988716704fd9d/src/transformers/activations.py#L45<|||||>> 

I'd be happy to put forward something to that effect in the next few days<|||||>Hi @mgoldey did you have time to work on this? It looks like it's going to be needed in the PR mentioned above as well, so we can take over on the implementation of the custom function Patrick mentioned you don't have time :-)<|||||>> Hi @mgoldey did you have time to work on this? It looks like it's going to be needed in the PR mentioned above as well, so we can take over on the implementation of the custom function Patrick mentioned you don't have time :-)

Feel free to jump in if you have time. I'll notify if I wind up being free, but I had to switch directions from time-sensitive projects.<|||||>Hi @mgoldey, I couldn't push to your branch, so I opened a fresh PR at #15180. I put you as co-author.<|||||>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.<|||||>Moot - resolved by #15180