Add Gated-SiLU to T5 (#17420)
* Add gated-silu to t5 architecture to support UL2
* Fix error message
* formatting
* formatting again
* refactor
* fix classnames in _init_weights
* remove is_gated
* add test
* fix test
* Try without the test?
* Add back the test.
* Improve error message.
Co-authored-by: Daniel Hesslow <daniel@lighton.ai>
