EncoderDecoder Cross Attention Generation Output Shape does not match Documentation
## Environment info
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- `transformers` version: 4.5.1
- Platform: Linux-4.15.0-143-generic-x86_64-with-glibc2.27
- Python version: 3.9.4
- PyTorch version (GPU?): 1.7.1 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: Yes (v100)
- Using distributed or parallel set-up in script?: No

### Who can help
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- encoderdecoder/text generation: @patrickvonplaten, @patil-suraj

## Information

Model I am using: EncoderDecoder with BERT

The problem arises when using:
* [x] the official example scripts: slightly modified/extended, see below

The tasks I am working on is:
* [x] my own task or dataset: just an example sentence from the docs

## To reproduce

Steps to reproduce the behavior:

1. Start with the example script from the [EncoderDecoder forward documenation](https://huggingface.co/transformers/model_doc/encoderdecoder.html#transformers.EncoderDecoderModel.forward)
2. Remove model training and model saving and loading steps (not relevant) and configure model to return attentions
3. Check shapes of cross attention outputs of generation and forward

```python
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, output_attentions=True)
forward_cross_attentions = outputs.cross_attentions

# As described in the docs the shapes are:
# "Tuple of torch.FloatTensor (one for each layer) of shape (batch_size, num_heads, sequence_length, sequence_length)"" (here (1,12,8,8))
print(f"Elements in forward cross attention: {len(forward_cross_attentions)}")
# Yields: Elements in forward cross attention: 12
print(f"Shapes in forward cross attention: {[fca.shape for fca in forward_cross_attentions]}")
# Yields: Shapes in forward cross attention: [torch.Size([1, 12, 8, 8]), torch.Size([1, 12, 8, 8]), torch.Size([1, 12, 8, 8]), torch.Size([1, 12, 8, 8]), torch.Size([1, 12, 8, 8]), torch.Size([1, 12, 8, 8]), torch.Size([1, 12, 8, 8]), torch.Size([1, 12, 8, 8]), torch.Size([1, 12, 8, 8]), torch.Size([1, 12, 8, 8]), torch.Size([1, 12, 8, 8]), torch.Size([1, 12, 8, 8])]


# generation
generated = model.generate(input_ids, decoder_start_token_id=model.config.decoder.pad_token_id, return_dict_in_generate=True, output_attentions=True)
generated_cross_attentions = generated.cross_attentions

# generated_cross_attentions contains 19 elements, maybe one for each generation step (generated.sequences has 20 elements)?
print(f"Elements in generation cross attention: {len(generated_cross_attentions)}")
# Yields: Elements in generation cross attention: 19

# All of the contained cross attentions have shape (1,12,1,8)
for cross_attention in generated_cross_attentions:
    print(f"Shapes in generation cross attention: {[gca.shape for gca in cross_attention]}")
# Yields: Shapes in generation cross attention: [torch.Size([1, 12, 1, 8]), torch.Size([1, 12, 1, 8]), torch.Size([1, 12, 1, 8]), torch.Size([1, 12, 1, 8]), torch.Size([1, 12, 1, 8]), torch.Size([1, 12, 1, 8]), torch.Size([1, 12, 1, 8]), torch.Size([1, 12, 1, 8]), torch.Size([1, 12, 1, 8]), torch.Size([1, 12, 1, 8]), torch.Size([1, 12, 1, 8]), torch.Size([1, 12, 1, 8])] (repeated 19 times)
```

Furthermore, if `num_beams>1` all `num_beams*batch_size` cross attentions are returned even if `num_return_sequences == 1`.

```python
# continued from above...
# generation
generated = model.generate(input_ids, decoder_start_token_id=model.config.decoder.pad_token_id, return_dict_in_generate=True, output_attentions=True, num_beams=10)
generated_cross_attentions = generated.cross_attentions

# generated_cross_attentions contains 19 elements, maybe one for each generation step (generated.sequences has 20 elements)?
print(f"Elements in generation cross attention: {len(generated_cross_attentions)}")

# All of the contained cross attentions have shape (10,12,1,8)
for cross_attention in generated_cross_attentions:
    print(f"Shapes in generation cross attention: {[gca.shape for gca in cross_attention]}")

print(f"Shape of the generated sequences: {generated.sequences.shape}")
Yields: torch.Size([1, 20])
```

## Expected behavior

A `Tuple of torch.FloatTensor (one for each layer) of shape (batch_size, num_heads, gen_sequence_length, sequence_length)`. Ideally the cross attentions batch size should match the batch size of the `generated.sequences`.

## Work arounds

Stacking the tuples and then concatting along the dimension which is 1 like this:
```python
torch.cat([torch.stack(ca) for ca in generated_cross_attentions], dim=-2)
```
yields such a tensor of the correct shape, is that the correct way to assemble it?

For the batch size issue I haven't found a work around yet. Is it possible to retain the beam indices of the selected beams from `generate`? `output_scores` is no help, because it has the same shape as `generated.sequences`.

Any help, ideas or pointers how to work around this are highly appreciated.
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