Fixing support `batch_size` and `num_return_Sequences` in `text-generation` pipeline
Author: Narsil
Pull request: yes

# What does this PR do?

And `text2text-generation` too.

The bug was caused by the batch_size containing both the incoming batch
**and** the generated `num_sequences`.

The fix simply consists into splitting both of these again into
different dimensions.

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Fixes #15316


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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Friendly ping @LysandreJik