🚨🚨🚨 Use default ignore index in Luke
# What does this PR do?

As discussed in #22981, the `ignore_index` for Luke should be the same as all models in Transformers, even if it does not match the original authors implementation.

This is breaking but needed to align all models to have the same API.

Fixes #22981
_The documentation is not available anymore as the PR was closed or merged._