Fix FlaxDataCollatorForT5MLM for texts with "<pad>"
If the input text contains `"<pad>"` (true for C4/en dataset) T5 tokenizer assigns it index 0.
Collator code (line 371) assumes that only tokens with `id > 0` are the text tokens and the rest are sentinel ids (`-1`), which turns out not to be true for this particular case.
This causes the line to throw an error similar to

```
input_ids = input_ids_full[input_ids_full > 0].reshape((batch_size, -1))
*** ValueError: cannot reshape array of size 1040383 into shape (8192,newaxis)
```

This modification should also allow to use padding in T5 pre-training.

@patil-suraj (note edited by @stas00 to tag the correct maintainer for flax)
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15443). All of your documentation changes will be reflected on that endpoint.<|||||>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/master/CONTRIBUTING.md) are likely to be ignored.<|||||>@patil-suraj , cold you please review this PR? It's very short<|||||>Oh we have a similar PR here from @patrickvonplaten

https://github.com/huggingface/transformers/pull/15835<|||||>Great!