Don't compute metrics in LM examples on TPU
# What does this PR do?

This PR removes the accuracy computation in the LM examples on TPU, as TPUs can't handle the accumulation of the model logits.

Patches #16005 A better fix would be to be able to gather those logits on TPU but waiting for advice from the pytorch XLA team on this.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thank you for the fix!