update the file list for doc testing
# What does this PR do?

As discussed in a previous email, I updated the file `utils/documentation_tests.txt`:
  - change the current 2 `.rst` files to the new `.mdx` files
  - add the pytorch modeling files that have no example issue (I tested them locally)

There are still some modeling files containing example issues, but we can fix them progressively.
I think it is a good start for 2022 :-)

## Who can review?

@LysandreJik @sgugger 


Oh wow that's quite a big change! Also cc @patrickvonplaten <|||||>Thanks for the change @ydshieh - I'm quite sure that most of the docstring tests in the mentioned files fail at the moment. Could you maybe do those changes: https://github.com/huggingface/transformers/pull/15031/files#r778215725 in your PR as well so that we can see which models have failing doc tests? :-)<|||||>> Thanks for the change @ydshieh - I'm quite sure that most of the docstring tests in the mentioned files fail at the moment. Could you maybe do those changes: https://github.com/huggingface/transformers/pull/15031/files#r778215725 in your PR as well so that we can see which models have failing doc tests? :-)

Hi, @patrickvonplaten , do you mean merge that PR into this one? Or just apply a specific change in that PR (if so, which one)?

I am not familiar with the test. The way I checked the doc examples locally is to extract those examples and run them (in some automatic way), and I only added the files without having doc example issue.

Thanks! <|||||>> Thanks for the change @ydshieh - I'm quite sure that most of the docstring tests in the mentioned files fail at the moment. Could you maybe do those changes: https://github.com/huggingface/transformers/pull/15031/files#r778215725 in your PR as well so that we can see which models have failing doc tests? :-)

Hi, I rebased my PR on your PR #15031. Let's see what it gives.

<|||||>@patrickvonplaten , in your PR, you added new lines in some doc examples, like

```
    >>> list(last_hidden_states.shape)
    {expected_output}
```

Is this necessary to make doctest work? Should I do the same for the doc examples in the files I added here?
(Could you point me a guide to deal with this doctest thing, please? Thanks)<|||||>Hey @ydshieh,

sorry for answering so late here. Let me discuss the doctests quickly internally and come back to you :-)<|||||>Hey @ydshieh,

We are currently working on adding a section that explains in detail how to test/check the doc tests. Would it be ok to close this PR for now and open a new one, once the documentation is up? <|||||>> Hey @ydshieh,
> 
> We are currently working on adding a section that explains in detail how to test/check the doc tests. Would it be ok to close this PR for now and open a new one, once the documentation is up?

Sure! Thanks for this update :-)