Fix usage of head masks by PT encoder-decoder models' generate() function (#11621)

* Add missing head masking for generate() function

* Add head_mask, decoder_head_mask and cross_attn_head_mask
into prepare_inputs_for_generation for generate() function
for multiple encoder-decoder models.

* Add test_genereate_with_head_masking

* [WIP] Update the new test and handle special cases

* make style

* Omit ProphetNet test so far

* make fix-copies

tests/test_modeling_prophetnet.py

@@ -1088,6 +1088,10 @@ class ProphetNetModelTest(ModelTesterMixin, GenerationTesterMixin, unittest.Test
         self.assertIsNotNone(encoder_hidden_states.grad)
         self.assertIsNotNone(encoder_attentions.grad)
 
+    def test_generate_with_head_masking(self):
+        """Generating with head_masking has not been implemented for ProphetNet models yet."""
+        pass
+
 
 @require_torch
 class ProphetNetStandaloneDecoderModelTest(ModelTesterMixin, GenerationTesterMixin, unittest.TestCase):