Add head_mask and decoder_head_mask to FSMT (#9819)
* Add {decoder_,}head_mask to fsmt_modeling.py
* Enable test_headmasking and some changes to docs
* Remove test_head_masking flag from fsmt test file
Remove test_head_masking flag from test_modeling_fsmt.py
since test_head_masking is set to be True by default (thus it is redundant to store).
* Merge master and remove test_head_masking = True
* Rebase necessary due to an update of jaxlib
* Remove test_head_masking=True in tests/test_modeling_fsmt.py
as it is redundant.
