Adding `num_return_sequences` support for text2text generation.
Author: Narsil
Pull request: yes

Co-Authored-By: Enze <pu.miao@foxmail.com>


# What does this PR do?

Superseed https://github.com/huggingface/transformers/pull/14411 and adds support
for multiple returned sequences from generate in t2t pipeline.

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Fixes # (issue)


## Before submitting
- [ ] 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/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
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      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
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