support `t5` for `text-generation` pipeline
# What does this PR do?

A tentative to support `T5` for `text-generation` pipeline. I don't really expect this PR to get merged as it is very hacky and IMO not a good idea to support `T5` for `text-generation` but I would love to have some insights on what we can potentially do to support `text-generation` pipeline for `T5`

Probably the fix would be also to implement `T5ForCausalLM` but not sure if this makes sense!
_The documentation is not available anymore as the PR was closed or merged._