Incorrect output fields names in docs
## Environment info
     
- `transformers` version: 3.2.0
- Platform: Linux-5.4.0-7642-generic-x86_64-with-glibc2.29
- Python version: 3.8.2
- PyTorch version (GPU?): 1.6.0 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: yes
- Using distributed or parallel set-up in script?: no

### Who can help
@sgugger

## Information

The model I am using (Bert, XLNet ...): Bart

The problem arises when using the official example scripts.

```Python
from transformers import BartModel, BartTokenizer

tokenizer = BartTokenizer.from_pretrained('facebook/bart-base')
model = BartModel.from_pretrained('facebook/bart-base', return_dict=True, 
                                   output_hidden_states=True, output_attentions=True)

inputs = tokenizer("Hello, my dog is cute", return_tensors="pt")
outputs = model(**inputs)

print(outputs.hidden_states)
```

This script results in the error `'Seq2SeqModelOutput' object has no attribute 'hidden_states'`.

## To reproduce

Steps to reproduce the behavior:

1. Run the script above.

## Expected behavior

My expectation was to get a set of hidden states for the model. But in fact, the model returns two sets of hidden states - one for the decoder and another one for the encoder. It can be observed by looking at the keys of the `outputs`:

```Python
>>> print(outputs.keys())
odict_keys(['last_hidden_state', 'decoder_hidden_states', 'encoder_last_hidden_state', 'encoder_hidden_states'])
```

The same is valid for attentions if I specify `output_attentions=True`:

```Python
>>> print(outputs.keys())
odict_keys(['last_hidden_state', 'decoder_hidden_states', 'decoder_attentions', 'encoder_last_hidden_state', 'encoder_hidden_states', 'encoder_attentions'])
```

My conclusion is that the documentation gives an incorrect description of the output fields.
Actually, the root of the problem might be related to the fact that the documentation states that the forward pass returns `BaseModelOutputWithPast` but in fact in returns `Seq2SeqModelOutput` ([source code](https://github.com/huggingface/transformers/blob/master/src/transformers/modeling_bart.py#L947-L955)).<|||||>Thanks for flagging! This should be fixed once the PR above is merged.<|||||>Solved by #7390