Fix doctest files fetch issue
Reverts huggingface/transformers#23271

Embarrassingly and unfortunately, the new job `tests_pr_documientation_tests` fails at the step `Get files to test` when the job is run on the `main` branch.

https://app.circleci.com/pipelines/github/huggingface/transformers/64235/workflows/54a99003-258e-4c2a-8366-b4461b3ec33f/jobs/794628/parallel-runs/0/steps/0-113

I will have to take a look - the log is not informative.
_The documentation is not available anymore as the PR was closed or merged._<|||||>As said offline I don't think we need to revert urgently, we can just ignore the red check on main.<|||||>@sgugger FYI The doctest PR also has some problem running on our GitHub Actions runner. See error below.
I will take a look, but this PR could be merged (without fixing the following issue) once you think the changes are good.


```bash
_____ ERROR collecting src/transformers/generation/configuration_utils.py ______
import file mismatch:
imported module 'transformers.generation.configuration_utils' has this __file__ attribute:
  /transformers/src/transformers/generation/configuration_utils.py
which is not the same as the test file we want to collect:
  /__w/transformers/transformers/src/transformers/generation/configuration_utils.py
HINT: remove __pycache__ / .pyc files and/or use a unique basename for your test file modules
```<|||||>> @sgugger FYI The doctest PR also has some problem running on our GitHub Actions runner. See error below. I will take a look, but this PR could be merged (without fixing the following issue) once you think the changes are good.
> 
> ```shell
> _____ ERROR collecting src/transformers/generation/configuration_utils.py ______
> import file mismatch:
> imported module 'transformers.generation.configuration_utils' has this __file__ attribute:
>   /transformers/src/transformers/generation/configuration_utils.py
> which is not the same as the test file we want to collect:
>   /__w/transformers/transformers/src/transformers/generation/configuration_utils.py
> HINT: remove __pycache__ / .pyc files and/or use a unique basename for your test file modules
> ```

@sgugger A short fix for this issue is given in [the last commit](https://github.com/huggingface/transformers/pull/23277/commits/bada5b3d616bff32f7440408428a4b9ed13c503b).

The reason is the file `conftest.py` has this line `from transformers.testing_utils import HfDoctestModule, HfDocTestParser` added in #23271. However, the `transformers` is installed during docker image build, which is different from the one when the CI is run.

This change should be applied to other workflow file, but it's rare that we have such imports in the codebase. I will do it in a separate PR.<|||||>Thanks for all the explanation!<|||||>Going to merge as the failing tests are irrelevant and I have tried to re-run for a few times.