Fix decode_input_ids to bare T5Model and improve doc
# What does this PR do?

* Fix 1: use the tokenizer to obtain the labels as tensors. `docs/source/en/model_doc/t5.mdx`
*  Fix 2: `src/transformers/models/t5/`
    * Present case: T5 prepends the decoder_input_ids with pad token. This preprocessing is handled internally by `T5ForConditionalGeneration` by shifting the labels to the right. 
    * Issue: This preprocessing needs to be done manually while using bare T5Model. This is missing from the example which uses bare T5Model.
    * Proposed Fix: Added a preprocessing step in the example so that the input matches with what T5 expects at its decoder. The PR reuses the `_shift_right()` method which is an internal function to T5. Please let me know if we can rename `_shift_right()` to `shift_right()` or if there is a better way to handle this.


## Before submitting
- [X] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


@sgugger @patrickvonplaten @patil-suraj

_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks for the fixes! <|||||>@patrickvonplaten Thanks for the review! Applied your suggestions. 