add support for new mcore ds features
@dimapihtar - can you add
drop_last_partial_validation_sequence = True and add_extra_token_to_sequence = True to a github actions test? > @dimapihtar - can you add drop_last_partial_validation_sequence = True and add_extra_token_to_sequence = True to a github actions test?

@ShriyaPalsamudram we have these values as True by default so I think all the tests are running in this way. One necessary consistency test will be to measure validation eval between legacy and mcore code paths, with all options turned on to eval on the entire validation dataset

You'll have to make sure that the loss and ppl computation is able to handle partial batches and partial sequences. NeMo already does this at least for the legacy code path, but you'll want to make sure there aren't any breaking changes.
# What does this PR do ?

Adds new mcore dataset features to NeMo.

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