Repository: huggingface/transformers (Python) — Bug Fix

Issue:
Error when passing encoder_outputs as tuple to EncoderDecoder models
## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->

- `transformers` version: 4.17.0.dev0
- Platform: Linux-5.13.0-27-generic-x86_64-with-glibc2.34
- Python version: 3.9.7
- PyTorch version (GPU?): 1.10.1+cu102 (True)
- Tensorflow version (GPU?): 2.7.0 (False)
- Flax version (CPU?/GPU?/TPU?): 0.3.6 (cpu)
- Jax version: 0.2.26
- JaxLib version: 0.1.75
- Using GPU in script?: yes
- Using distributed or parallel set-up in script?: no


### Who can help
@patrickvonplaten

## Information

In EncoderDecoder models one can pass `encoder_outputs` [as a tuple of Tensors ](https://github.com/jsnfly/transformers/blob/8ce133063120683018b214fe10d1449e4c2401da/src/transformers/models/encoder_decoder/modeling_encoder_decoder.py#L106). However, if you do that [this line](https://github.com/jsnfly/transformers/blob/8ce133063120683018b214fe10d1449e4c2401da/src/transformers/models/encoder_decoder/modeling_encoder_decoder.py#L549) will fail with
```python
AttributeError: 'tuple' object has no attribute 'last_hidden_state'
``` 
since the tuple isn't modified in the `forward` method.
So if it is a tuple, `encoder_outputs` could maybe wrapped in a `ModelOutput` class or something similar. Or handle the tuple somehow explicitly.

## On a slight tangent

I made a `SpeechEncoderDecoderModel` for the robust speech challenge: https://huggingface.co/jsnfly/wav2vec2-large-xlsr-53-german-gpt2. I found that adding the position embeddings of the decoder model to the outputs of the encoder model improved performance significantly (basically didn't work without it). 
This needs [small modifications](https://huggingface.co/jsnfly/wav2vec2-large-xlsr-53-german-gpt2/blob/main/training/model.py#L8) to the `__init__` and `forward` methods of the `SpeechEncoderDecoderModel`.

At the moment this seems to me too much of a "hack" to add it to the `SpeechEncoderDecoderModel` class generally (for example via a flag), because it may differ for different `decoder` models and probably also needs more verification. @patrickvonplaten showed some interest that this could be included in Transformers nonetheless. What do you think? 



Discussion:
Hey @jsnfly,

Regarding the first point - agree, it'd be good to check if the input is a tuple and if it is we can wrap it into a `ModelOutput` object. Would you be interested in opening a PR for this? :-)

Regarding the 2nd point - that's very interesting (cc @sanchit-gandhi). Also makes a lot of sense since ASR by itself is monotonic so knowing the order of words to transcribe together with the encoder speech frames seems like a sensible design architecture. Thanks a lot for sharing this here!
The embedding hack is a really neat find - nice one @jsnfly! It's something we're going to take a look into in our ASR experiments! It seems like it could help with alignment in a much cleaner and more compact way than the encoder-decoder cross-attention mechanism.

Resolving patch:
diff --git a/src/transformers/models/encoder_decoder/modeling_encoder_decoder.py b/src/transformers/models/encoder_decoder/modeling_encoder_decoder.py
--- a/src/transformers/models/encoder_decoder/modeling_encoder_decoder.py
+++ b/src/transformers/models/encoder_decoder/modeling_encoder_decoder.py
@@ -22,7 +22,7 @@
 from torch.nn import CrossEntropyLoss
 
 from ...configuration_utils import PretrainedConfig
-from ...modeling_outputs import Seq2SeqLMOutput
+from ...modeling_outputs import BaseModelOutput, Seq2SeqLMOutput
 from ...modeling_utils import PreTrainedModel
 from ...utils import add_start_docstrings, add_start_docstrings_to_model_forward, logging, replace_return_docstrings
 from ..auto.configuration_auto import AutoConfig
@@ -494,6 +494,8 @@ def forward(
                 return_dict=return_dict,
                 **kwargs_encoder,
             )
+        elif isinstance(encoder_outputs, tuple):
+            encoder_outputs = BaseModelOutput(*encoder_outputs)
 
         encoder_hidden_states = encoder_outputs[0]
 
diff --git a/src/transformers/models/speech_encoder_decoder/modeling_speech_encoder_decoder.py b/src/transformers/models/speech_encoder_decoder/modeling_speech_encoder_decoder.py
--- a/src/transformers/models/speech_encoder_decoder/modeling_speech_encoder_decoder.py
+++ b/src/transformers/models/speech_encoder_decoder/modeling_speech_encoder_decoder.py
@@ -22,7 +22,7 @@
 from torch.nn import CrossEntropyLoss
 
 from ...configuration_utils import PretrainedConfig
-from ...modeling_outputs import Seq2SeqLMOutput
+from ...modeling_outputs import BaseModelOutput, Seq2SeqLMOutput
 from ...modeling_utils import PreTrainedModel
 from ...utils import add_start_docstrings, add_start_docstrings_to_model_forward, logging, replace_return_docstrings
 from ..auto.configuration_auto import AutoConfig
@@ -514,6 +514,8 @@ def forward(
                 return_dict=return_dict,
                 **kwargs_encoder,
             )
+        elif isinstance(encoder_outputs, tuple):
+            encoder_outputs = BaseModelOutput(*encoder_outputs)
 
         encoder_hidden_states = encoder_outputs[0]
 
diff --git a/src/transformers/models/vision_encoder_decoder/modeling_vision_encoder_decoder.py b/src/transformers/models/vision_encoder_decoder/modeling_vision_encoder_decoder.py
--- a/src/transformers/models/vision_encoder_decoder/modeling_vision_encoder_decoder.py
+++ b/src/transformers/models/vision_encoder_decoder/modeling_vision_encoder_decoder.py
@@ -22,7 +22,7 @@
 from torch.nn import CrossEntropyLoss
 
 from ...configuration_utils import PretrainedConfig
-from ...modeling_outputs import Seq2SeqLMOutput
+from ...modeling_outputs import BaseModelOutput, Seq2SeqLMOutput
 from ...modeling_utils import PreTrainedModel
 from ...utils import add_start_docstrings, add_start_docstrings_to_model_forward, logging, replace_return_docstrings
 from ..auto.configuration_auto import AutoConfig
@@ -466,6 +466,8 @@ def forward(
                 return_dict=return_dict,
                 **kwargs_encoder,
             )
+        elif isinstance(encoder_outputs, tuple):
+            encoder_outputs = BaseModelOutput(*encoder_outputs)
 
         encoder_hidden_states = encoder_outputs[0]
 


Test patch:
diff --git a/tests/encoder_decoder/test_modeling_encoder_decoder.py b/tests/encoder_decoder/test_modeling_encoder_decoder.py
--- a/tests/encoder_decoder/test_modeling_encoder_decoder.py
+++ b/tests/encoder_decoder/test_modeling_encoder_decoder.py
@@ -142,6 +142,22 @@ def check_encoder_decoder_model(
             outputs_encoder_decoder["encoder_last_hidden_state"].shape, (input_ids.shape + (config.hidden_size,))
         )
 
+        # Test passing encoder_outputs as tuple.
+        encoder_outputs = (encoder_hidden_states,)
+        outputs_encoder_decoder = enc_dec_model(
+            encoder_outputs=encoder_outputs,
+            decoder_input_ids=decoder_input_ids,
+            attention_mask=attention_mask,
+            decoder_attention_mask=decoder_attention_mask,
+        )
+
+        self.assertEqual(
+            outputs_encoder_decoder["logits"].shape, (decoder_input_ids.shape + (decoder_config.vocab_size,))
+        )
+        self.assertEqual(
+            outputs_encoder_decoder["encoder_last_hidden_state"].shape, (input_ids.shape + (config.hidden_size,))
+        )
+
     def check_encoder_decoder_model_from_pretrained_using_model_paths(
         self,
         config,