[T5 3B Covid 19] Adapt T5 TF conversion script to handle covid-19 3b t5
This PR shows which changes were necessary to convert the 3B (and 11B) T5 model from this issue: https://github.com/huggingface/transformers/tree/adapt_t5_for_covid_19_3b to PyTorch. 

It might be possible that the official T5 library has changed in which case this code might be useful again. 
For now, this PR stays a draft though, but can be cleaned and merged if more T5 Conversion issues arise. 

Pinging @sshleifer @thomwolf for notification.
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