Fix TF Longformer
# What does this PR do?

This PR aims to fix the TF Longformer version in order to make it graph compliant. As seen offline with @patrickvonplaten `all_global_attentions` now is added in the output when `output_attentions=True`. The global attentions are filled with zeros in case `is_global_attn` is False (see line 897 in `TFLongformerSelfAttention`.

# Fix issue
#9333 

I have already ran the slow tests as well and they all pass!