[M2M100, XGLM] fix positional emb resize
# What does this PR do?

The sinusoidal position embeddings in M2M100 and XGLM are not correctly resized as the `max_pos` is computed in-correctly. Take into account the `past_key_values_length` when computing `max_pos`.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Looks good to me - a bit surprising to me that no one saw this earlier