Fixing a hard to trigger bug for `text-generation` pipeline.
Author: Narsil
Pull request: yes

# What does this PR do?
This PR readds sending `attention_mask` to the `generate` function.

In order to trigger the bug ones would need:

- To use the pipeline in `batch_size>1` mode.
- Use a model that did not configure `pad_token_id`. (if it is configured, than generate just recovers the attention mask gracefully)

Then the `generate` function would not be able to recover the attention mask, warn
about it but still generate something (most likely incorrect)

Since the pipeline most likely already generated the `attention_mask` we might as well
send it along to `generate`.

@sgugger 

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Fixes # (issue)


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Comments:
_The documentation is not available anymore as the PR was closed or merged._