Removing duplicated code for Translation,Summarization and Text2TextGeneration pipelines
Author: Narsil
Pull request: yes

# What does this PR do?

`TranslationPipeline`, `SummarizationPipeline` and `Text2TextGenerationPipeline` share quite a bit of code for the generation
part. This PR aims to remove that code duplication to prevent future errors in argument handling while preserving, documentation
for all methods and functions and the full behavior.

Translation and Summarization now inherit from Text2TextGenerationPipeline. They retain their own docstrings to be more readable in the docs.

New function `check_inputs` has appeared which does all the current variation between the 3 classes, basically by raising different warnings
based on inputs and underlying model config.

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## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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## Who can review?

@LysandreJik
@sgugger

Edit: Sorry about the diff, just gave a look, it's totally unreadable mostly because I reordered the classes so that the Base classe (Text2TextGenerationPipeline) is before the subclasses.
I'll happily switch that back to make review on the actual code easier (and maybe change back later the order for cleaner code in the end)

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors which may be interested in your PR.

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 albert, bert, XLM: @LysandreJik
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Comments:
Feel free to merge whenever @Narsil