Add Cross attention to GPTNeo
### Feature request

I have been working on using GPTNeo as a decoder for an encoder-decoder model, and I was hoping to add cross attention to GPTNeo. 

### Motivation

For encoder-decoder model

### Your contribution

I would like to submit a PR for it but anyone is up to taking it 
@patil-suraj <|||||>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.

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