make the loss context manager easier to extend
Author: winglian
Pull request: yes

# What does this PR do?

Using ExitStack makes it easier to extend the functionality of the context manager wrapping the `model(inputs)` call. This can make it easier to add other things like logging contexts, checkpointing contexts, etc down the road.

e.g.

```python
class CustomTrainer(Trainer):
    def compute_loss_context_manager(self):
        ctx_stack = super(). compute_loss_context_manager()
        my_custom_ctx = CustomContext(...)
        ctx_stack.enter_context(my_custom_ctx)
        return ctx_stack
````

Fixes # (issue)


## Before submitting
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      Pull Request section?
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      to it if that's the case.
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      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@SunMarc @ArthurZucker 
Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

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Comments:
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