[trainer] add option to ignore keys for the train function too (#11719)
# What does this PR do?

This pull request adds the option of ignoring certain output keys for evaluation during the training phase. As of now, this option is only available for `predict` and `evaluation` methods of the `Trainer` class which can only be called after the training.

Fixes #11719

Changes:

1. Add a new parameter to the `Trainer.train` function called `ignore_keys_for_eval`.
2. Pass this to the `ignore_keys` parameter of the `trainer.evaluate` function that is already called within the `trainer.train` function.
2. Add the parameter to the docstring.