Bart: check if decoder_inputs_embeds is set (#13800)

In BartForConditionalGeneration.forward, if labels are provided,
   decoder_input_ids are set to the labels shifted to the right.
   This is problematic: if decoder_inputs_embeds is also set,
   the call to self.model, which eventually gets to BartDecoder.forward,
   will raise an error.
   The fix is quite simple, similar to what is there already in
   BartModel.forward. Mainly, we should not
   compute decoder_input_ids if decoder_inputs_embeds is provided.

Co-authored-by: Silviu Vlad Oprea <silviuvo@amazon.co.uk>

src/transformers/models/bart/modeling_bart.py

@@ -1291,7 +1291,7 @@ def forward(
         return_dict = return_dict if return_dict is not None else self.config.use_return_dict
 
         if labels is not None:
-            if decoder_input_ids is None:
+            if decoder_input_ids is None and decoder_inputs_embeds is None:
                 decoder_input_ids = shift_tokens_right(
                     labels, self.config.pad_token_id, self.config.decoder_start_token_id
                 )

src/transformers/models/bigbird_pegasus/modeling_bigbird_pegasus.py

@@ -2501,7 +2501,7 @@ def forward(
         return_dict = return_dict if return_dict is not None else self.config.use_return_dict
 
         if labels is not None:
-            if decoder_input_ids is None:
+            if decoder_input_ids is None and decoder_inputs_embeds is None:
                 decoder_input_ids = shift_tokens_right(
                     labels, self.config.pad_token_id, self.config.decoder_start_token_id
                 )