PegasusForCausalLM, analog to `ProphetNetForCausalLM`
# What does this PR do?

this PR is implementing PegasusForCausalLM for EncoderDecoder use like ProphetNetForCausalLM  and it follow work of BartForCausalLM #9128 
issue #9066 

for a organisation matter, each one is in its own PR. @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.