Adding `top_k` argument to `text-classification` pipeline.
Author: Narsil
Pull request: yes

# What does this PR do?

A lot of users are wondering why the API does not return results sorted.
This PR enables `transformers` to do that and enables functionnality
to users without causing any regression.

The API will simply override the default argument to get sorted results.

- Deprecate `return_all_scores` as `top_k` is more uniform with other
  pipelines, and a superset of what `return_all_scores` can do.
  BC is maintained though.
  `return_all_scores=True` -> `top_k=None`
  `return_all_scores=False` -> `top_k=1`

- Using `top_k` will imply sorting the results, but using no argument
  will keep the results unsorted for backward compatibility.

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Fixes # (issue)


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Comments:
_The documentation is not available anymore as the PR was closed or merged._

This PR should be cleaned up, it changed the behavior of the text classification pipeline and the documentation is not clear. Can we get more explanations about the way we should expect to modify the code to get the same behavior with the top_k arg or continue supporting the return_all_scores with the same output order?

Passing `return_all_scores=True` is not equivalent to `top_k=1` for example, and setting `top_k=n` is sorting the results which is not the same order as before this change. Please provide better documentation and some way to aleviate the pain of upgrading the transformers library by minimizing the incompatible changes.