Add the T5 seq2seq model

[EricLBuehler]

This PR implements our first Seq2Seq model, T5. Refs #384.

[github-actions[bot]]

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[brian-at-pieces]

It's been a while but is there any plan to merge this in soon? Especially interested with Google releasing T5Gemma https://huggingface.co/docs/transformers/en/model_doc/t5gemma. Thanks!