[T5 fp16] Fix fp16 in T5
This PR fixes the issue: #4287. 

- A test for T5 is added.
- The function self.invert_attention_mask included a if statement now so that no errors will occur when using the function in `fp16` mode.
> Bart doesn't use this method yet, but LGTM!

Yeah, I noticed that as well - it's Bert that is using it.