seq2seq BertGeneration model failed "ValueError: You have to specify either input_ids or inputs_embeds"
## 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: 4.4.0.dev0
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj

```
python examples/seq2seq/run_seq2seq.py \
    --model_name_or_path google/roberta2roberta_L-24_discofuse \
    --do_train \
    --do_eval \
    --task summarization \
    --train_file path_to_csv_or_jsonlines_file \
    --validation_file path_to_csv_or_jsonlines_file \
    --output_dir /tmp/tst-summarization \
    --per_device_train_batch_size=4 \
    --per_device_eval_batch_size=4 \
    --overwrite_output_dir \
    --predict_with_generate \
    --max_train_samples 500 \
    --max_val_samples 500
```

path_to_csv_or_jsonlines_file:
```
text,summary
google map, gg map
google translate, gg translate
```

t5-small works perfectly. But BertGeneration model has the following error



error:
```
  File "/Users/gyin/Documents/working/transformers/src/transformers/models/bert_generation/modeling_bert_generation.py", line 361, in forward
    raise ValueError("You have to specify either input_ids or inputs_embeds")
ValueError: You have to specify either input_ids or inputs_embeds

```

hi @gyin-ai 

Thank you for reporting the issue. The `run_seq2seq.py` currently does not work for encoder-decoder models. This is because the encoder-decoder models expect both `decoder_input_ids` and `labels` whereas the script only passes the `labels`. Which is causing the above error.

You could refer to this [notebook](https://colab.research.google.com/github/patrickvonplaten/notebooks/blob/master/RoBERTaShared_for_BBC_XSum.ipynb) to see how to use `Trainer` for encoder-decoder models. Also, you easily adapt the `run_seq2seq.py` script for this, I think you'll only need to change the  data collator [here](https://github.com/huggingface/transformers/blob/master/examples/seq2seq/run_seq2seq.py#L521) to return both the `labels` and `decoder_input_ids`<|||||>@patil-suraj can I ask whether `batch["decoder_input_ids"]` should be `inputs.input_ids` instead of `outputs.input_ids`?

```
def process_data_to_model_inputs(batch):                                                               
    # Tokenizer will automatically set [BOS] <text> [EOS]                                               
    inputs = tokenizer(batch["document"], padding="max_length", truncation=True, max_length=encoder_max_length)
    outputs = tokenizer(batch["summary"], padding="max_length", truncation=True, max_length=decoder_max_length)
                                                                                                        
    batch["input_ids"] = inputs.input_ids                                                               
    batch["attention_mask"] = inputs.attention_mask                                                     
    batch["decoder_input_ids"] = outputs.input_ids                                                      
    batch["labels"] = outputs.input_ids.copy()                                                          
    # mask loss for padding                                                                             
    batch["labels"] = [                                                                                 
        [-100 if token == tokenizer.pad_token_id else token for token in labels] for labels in batch["labels"]
    ]                     
    batch["decoder_attention_mask"] = outputs.attention_mask                                                                              
                                                                                                         
    return batch  
```

here is the example from EncoderDecoderModel
```
>>> from transformers import EncoderDecoderModel, BertTokenizer
>>> import torch

>>> tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
>>> model = EncoderDecoderModel.from_encoder_decoder_pretrained('bert-base-uncased', 'bert-base-uncased') # initialize Bert2Bert from pre-trained checkpoints

>>> # forward
>>> input_ids = torch.tensor(tokenizer.encode("Hello, my dog is cute", add_special_tokens=True)).unsqueeze(0)  # Batch size 1
>>> outputs = model(input_ids=input_ids, decoder_input_ids=input_ids)

>>> # training
>>> outputs = model(input_ids=input_ids, decoder_input_ids=input_ids, labels=input_ids)
```<|||||>The `labels` and `decoder_input_ids` always correspond to output. so it should be `outputs.input_ids`