Add inputs_embeds functionality when generating with GPT-Neox 
This PR extends https://github.com/huggingface/transformers/pull/21405  and #21889  by @gante  to GPTNeox models (which also includes the recent Pythia Suite models), , making it accept inputs_embeds when generating.

## Who can Review?
@gante @sgugger 
_The documentation is not available anymore as the PR was closed or merged._