add t5 for text generation
# What does this PR do?

Add support for `T5ForConditionalGeneration` to the text generation pipeline


## Who can review?

@LysandreJik  @patrickvonplaten 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_18326). All of your documentation changes will be reflected on that endpoint.<|||||>Thanks for opening the PR @sam-h-bean, can you explain a bit more why you require this change? Can't you use the text-to-text pipeline in your application to leverage T5 instead?

I'm **not** in favor of this PR as is because `T5ForConditionalGeneration` is clearly a different architecture (and input/output format) then `...CausalLM` which is GPT-like style<|||||>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.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.