Add mLUKE
# What does this PR do?

Reopen the following PR:
https://github.com/huggingface/transformers/pull/14570

I have reflected the comments in the previous PR (add `# Copied from ...` comments, replace `assert` with `ValueError`).
I have also added few tests to make sure `LukeTokenizer` and `MLukeTokenizer` correctly handle padding with entity inputs.

## Who can review?
@NielsRogge  @LysandreJik  @sgugger 
It looks like there a quite a few failures in the Luke tests. You can see them locally with:
```
pytest tests/test_modeling_luke.py
```
from the root repo.<|||||>Thanks, I’ve fixed that.
I still see some tests failed in CI (`run_tests_tf`), but I guess it is not from the changes in this PR?<|||||>Yes, they're timing out for some reason, it's not a failure due to your PR.

Thanks a lot for adding this new model!