BART & FSMT: fix decoder not returning hidden states from the last layer (#8597)

* Fix decoder not returning hidden states from the last layer

* Resolve conflict

* Change the way to gather hidden states

* Add decoder hidden states test

* Make pytest and black happy

* Remove redundant line

* remove new line
Co-authored-by: Stas Bekman <stas00@users.noreply.github.com>

src/transformers/models/bart/modeling_bart.py

@@ -610,6 +610,12 @@ class BartDecoder(nn.Module):
                 all_self_attns += (layer_self_attn,)
                 all_cross_attentions += (layer_cross_attn,)
 
+        # add hidden states from the last decoder layer
+        if output_hidden_states:
+            x = x.transpose(0, 1)
+            all_hidden_states += (x,)
+            x = x.transpose(0, 1)
+
         if self.layer_norm:  # if config.add_final_layer_norm (mBART)
             x = self.layer_norm(x)
 

src/transformers/models/fsmt/modeling_fsmt.py

@@ -692,6 +692,12 @@ class FSMTDecoder(nn.Module):
                 all_self_attns += (layer_self_attn,)
                 all_cross_attns += (layer_cross_attn,)
 
+        # add hidden states from the last decoder layer
+        if output_hidden_states:
+            x = x.transpose(0, 1)
+            all_hidden_states += (x,)
+            x = x.transpose(0, 1)
+
         # Convert to standard output format: (seq_len, BS, model_dim) -> (BS, seq_len, model_dim)
         x = x.transpose(0, 1)
         encoder_hidden_states = encoder_hidden_states.transpose(0, 1)

tests/test_modeling_common.py

@@ -659,12 +659,14 @@ class ModelTesterMixin:
 
             with torch.no_grad():
                 outputs = model(**self._prepare_for_class(inputs_dict, model_class))
-            hidden_states = outputs["hidden_states"] if "hidden_states" in outputs else outputs[-1]
+
+            hidden_states = outputs.encoder_hidden_states if config.is_encoder_decoder else outputs.hidden_states
 
             expected_num_layers = getattr(
                 self.model_tester, "expected_num_hidden_layers", self.model_tester.num_hidden_layers + 1
             )
             self.assertEqual(len(hidden_states), expected_num_layers)
+
             if hasattr(self.model_tester, "encoder_seq_length"):
                 seq_length = self.model_tester.encoder_seq_length
                 if hasattr(self.model_tester, "chunk_length") and self.model_tester.chunk_length > 1:
@@ -677,6 +679,19 @@ class ModelTesterMixin:
                 [seq_length, self.model_tester.hidden_size],
             )
 
+            if config.is_encoder_decoder:
+                hidden_states = outputs.decoder_hidden_states
+
+                self.assertIsInstance(hidden_states, (list, tuple))
+                self.assertEqual(len(hidden_states), expected_num_layers)
+                seq_len = getattr(self.model_tester, "seq_length", None)
+                decoder_seq_length = getattr(self.model_tester, "decoder_seq_length", seq_len)
+
+                self.assertListEqual(
+                    list(hidden_states[0].shape[-2:]),
+                    [decoder_seq_length, self.model_tester.hidden_size],
+                )
+
         config, inputs_dict = self.model_tester.prepare_config_and_inputs_for_common()
 
         for model_class in self.all_model_classes: