[T5] Add t5 download script

[patrickvonplaten]

What does this PR do?

There have been a lot of issues on converting any of the official T5 checkpoints to HF. All original T5 checkpoints are stored on google cloud bucked and it's not always obvious how to download them. I think it makes sense to provide a bash script to the T5 folder that can help with this.

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[patrickvonplaten]

cc @peregilk @stefan-it

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[sgugger]

Thanks for adding it! For discovery, I think you should add a link to it in the documentation of T5.

[patil-suraj]

Thank you for adding this!