Transformers: Add T5 model

[astariul-colanim]

🌟New model addition

Model description

Google released paper + code + dataset + pre-trained model about their new T5, beating state-of-the-art in 17/24 tasks.

Paper link

Open Source status

• [x] the model implementation and weights are available: Official codebase

[tnlin]

+1, it is a very impressive work

[Quetzalcohuatl]

https://github.com/google-research/text-to-text-transfer-transformer

However i would prefer seeing Albert implemented before T5.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[happy-machine]

Please

[elfprince13]

It's not super-well documented, but it's clearly present:
https://github.com/huggingface/transformers/blob/dc17f2a1110aed8d1729e77b0619601e3d96b84e/src/transformers/modeling_tf_t5.py