Add T5 model
# 🌟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](https://arxiv.org/pdf/1910.10683.pdf)

## Open Source status

* [x] the model implementation and weights are available: [Official codebase](https://github.com/google-research/text-to-text-transfer-transformer)

+1, it is a very impressive work<|||||>https://github.com/google-research/text-to-text-transfer-transformer

However i would prefer seeing Albert implemented before T5.<|||||>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.
<|||||>Please<|||||>It's not super-well documented, but it's clearly present: 
https://github.com/huggingface/transformers/blob/dc17f2a1110aed8d1729e77b0619601e3d96b84e/src/transformers/modeling_tf_t5.py