Implementation of BlenderbotForCausalLM
# What does this PR do?
implementing BlenderbotForCausalLM for EncoderDecoder use, like ProphetNetForCausalLM

Fixes # (issue)
#9066 
#9128 

@patrickvonplaten

Hey @sadakmed, sorry I might have been a bit unclear here. Could you instead of opening new PRs add your added code directly to your PR here: https://github.com/huggingface/transformers/pull/9128? The `#Copy from` statements force us to have all the code in the same PR.

Also we need a decoder-only test for this.