Add `cross_attentions` to the output of TensorFlow encoder-decoder models
# 🚀 Feature request

TensorFlow encoder-decoder models cannot return `cross_attentions` as do their PyTorch counterparts.

## Motivation
It would be nice to narrow the gap between PyTorch and Tensorflow implementations.

## Your contribution
I've been working on PR fixing this issue.

## Reviewers
@jplu and whoever else within the community
This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.