Fixes all hidden states output in FlaxT5
Fixes #23960

@sanchit-gandhi 
_The documentation is not available anymore as the PR was closed or merged._<|||||>need to fix the tests<|||||>Hmm okay so the PyTorch model is also missing this, so we'd need to update it here. Continuing the discussion in the issue!<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>Based on the discussion in the issue: https://github.com/huggingface/transformers/issues/23960#issuecomment-1579140627, we should probably not update the Flax and PyTorch T5 models since this would be a surprise breaking change to what is one of the most popular models in the lib. Feel free to make these changes locally @ztjhz if you need all the hidden states! Otherwise we can close this one for now

cc @amyeroberts 