[Doctest] Add configuration_transfo_xl.py
# What does this PR do?

Add `configuration_transfo_xl.py` to `utils/documentation_tests.txt` for doctest.

Based on issue  #19487

@ydshieh could you please take a look at it?
Thanks =)

_The documentation is not available anymore as the PR was closed or merged._<|||||>@thliang01 Could you try to resolve the conflict :-) thank you 🙏 <|||||>Hi @thliang01 I tried to fix the conflict and make it clean. It should work now - I will merge once the CI are all good. Thank you again for your contribution 👍 💯 !