Add head_mask and decoder_head_mask to PyTorch LED (#9856)

* Add {decoder_,}head_mask to LED

* Fix create_custom_forward signatue in encoder

* Add head_mask to longformer

* Add head_mask to longformer to fix dependencies
of LED on Longformer.

* Not working yet

* Add mising one input in longofrmer_modeling.py

* make fix-copies

tests/test_modeling_common.py

@@ -473,7 +473,6 @@ class ModelTesterMixin:
                 arg_names = [*signature.parameters.keys()]
                 if "decoder_head_mask" in arg_names:  # necessary diferentiation because of T5 model
                     inputs["decoder_head_mask"] = head_mask
-
             outputs = model(**inputs, return_dict=True)
 
             # Test that we can get a gradient back for importance score computation