T5ForConditionalGeneration with BetterTransformer
### Feature request

Got this error when trying out BetterTransformer:
"The Better Transformers implementation for the model T5ForConditionalGeneration has not been implemented yet. Please open an issue requesting the addition of this model with its `BetterTransformer`implementation."

### Motivation

I would like to speed up some text2text models with BetterTransformer

### Your contribution

I can at least test the feature once it is ready
I think it needs to be implemented in optimum, so you should move the issue there. cc @younesbelkada <|||||>Moving to optimum