Adding a new `return_full_text` parameter to TextGenerationPipeline.
Author: Narsil
Pull request: yes

# What does this PR do?

For text-generation, it's sometimes used as prompting text.
In that context, prefixing `generated_text` with the actual input
forces the caller to take an extra step to remove it.

The proposed change adds a new parameter (for backward compatibility).
`return_full_text` that enables the caller to prevent adding the prefix.


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## Before submitting
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@LysandreJik
@patrickvonplaten

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Comments:
Don't mind the failing test, you can merge when ready.