Correct wrong function signatures on the docs website
# What does this PR do?

Trying to address #13171.


## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


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@sgugger @patrickvonplaten 

After a series of investigations, here is the concluding matrix:

| Env                       | Python version | Sphinx version | Correctness     |
|-------------------- |------------------|-----------------|-----------------|
| Circle CI Docker Image   |         3.6           |         3.2.1         |       X    |
| Circle CI Docker Image   |         3.6           |         3.5.4         |       X    |
| Circle CI Docker Image   |    3.7(3.7.11)    |         3.2.1         |       X    |
| Circle CI Docker Image   |    3.7(3.7.11)    |         3.5.4         |      *O [Artifact](https://258083-155220641-gh.circle-artifacts.com/0/docs/_build/html/main_classes/trainer.html)  |
| Ubuntu 18.04 Anaconda |         3.6.13      |         3.2.1         |       X    |
| Ubuntu 18.04 Anaconda |         3.7.11      |         3.2.1         |       O    |
| Ubuntu 18.04 Anaconda |         3.8.5        |         3.2.1         |       O    |

X: 
`model: torch.nn.modules.module.Module = None` (Union and PreTrainedModel missing)
O:
`model: Union[transformers.modeling_utils.PreTrainedModel, torch.nn.modules.module.Module] = None` (Correct)
*O: 
`Optional[Union[transformers.modeling_utils.PreTrainedModel, torch.nn.modules.module.Module]] = None`
(An `Optional` type hint was added by Sphinx which wasn't defined in the code, maybe inferred from the default value `None`)

As shown in the above matrix, Sphinx (3.2.1 & 3.5.4) with python 3.6 failed to generate correct html in testing environments, should we upgrade CI/CD environments all together to python 3.7 in order to keep the consistency?

Besides, I noticed that in `docs/source/conf.py`, the release version is `4.7.0`, which isn't the latest version `4.9.2` , should this also need to be updated?
<|||||>@sgugger correctly mentions I merged this without the last comment being taken into account - Sorry about that, Sylvain is pushing directly on `master` with the comment's request.<|||||>I actually made a PR in #13337 :-) <|||||>This had an unintended side-effect: the search functionality doesn't seem to be working anymore on huggingface.co/transformers.

I tracked the issue to Sphinx version v3.4.0. Checking out your useful table @qqaatw, switching back to v3.2.1 with Python v3.7x would be the second best choice?<|||||>@LysandreJik I've checked the search functionality, it's not working indeed. 

As you said, maybe we should switch back Sphinx's version to v3.2.1 but not with Python v3.7.11 because Sphinx v3.2.1 with Python v3.7.11 provided by CircleCI docker image seems not working either. It's weird though as I tested this combination on my machine (Ubuntu 18.04 Anaconda) and the output was correct.

I think another try would be using [Next-gen language images](https://circleci.com/docs/2.0/circleci-images/#next-gen-language-images). According to what the website states, these images are faster to build and have improved reliability and stability. Perhaps switching to this one can solve this problem.<|||||>Since we are exploring a move away from sphinx anyway, we will revert this commit for now to re-enable the search. If we end up not moving away from sphinx we can explore more which image to pick and which versions to use, but in the meantime, it's more important to have the search enabled than the sometime wrong signatures.<|||||>Got it. Sorry for the inconvenience.<|||||>No worries!