Fix some docs what layerdrop does
# What does this PR do?

Fix some configuration docs what layerdrop does. Copy from `configuration_opt.py` and reset defaults which is same as init values.

Fixes # (issue)
https://github.com/huggingface/transformers/issues/23351

## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@sgugger, @stevhliu, @MKhalusova and @sanchit-gandhi 


_The documentation is not available anymore as the PR was closed or merged._<|||||>> Thanks for your fix! Just have one syntax comment to propagate on all docs.

Thanks for your suggestion! I will add more commits later.