Add cross attentions to TFGPT2Model
# What does this PR do?

Add cross attention to TFGPT2.

This was previously done in #13222, but we decided to move this to a new PR.
I also added `TFGPT2EncoderDecoderModelTest` with `test_bert2gpt2_summarization`.
Just correct the line about 

```
"""Not working, because pt checkpoint has `encoder.encoder.layer...` while tf model has `encoder.bert.layer...`.
```

For tf here, it has `encoder.bert.encoder.layer...` instead.<|||||>Sorry, I tried fixing some merge conflicts, but I think this introduced a new error. @ydshieh could you maybe quickly go into the PR again and fix those last tests? :-) The PR looks good for me otherwise!<|||||>> 
> 
> Sorry, I tried fixing some merge conflicts, but I think this introduced a new error. @ydshieh could you maybe quickly go into the PR again and fix those last tests? :-) The PR looks good for me otherwise!

It's OK now :)