Update T5gemma (#39210)
* bug fix: add vocab_size to t5gemmaconfig for pipeline.
* Update checkpoint placeholder
* minor change
* minor change
* minor change: update example.
* fix: add vocab_size as an explict arg.
* buf fix:
remove vocab_size verification; instead, re-set encoder/decoder vocab size.
Note, in t5gemma, vocab size of encoder/decoder shoud be always the same.
* add `add_generation_prompt` for message preprocessing.
