[Refactor] Splitting pipelines.py into its own module.
Author: Narsil
Pull request: yes

# What does this PR do?

Moves various pipelines into their own files.
`pipelines.py` was 3k+ lines of code with feels a bit too much. To go along with the `models` split
into various files, splitting into a cleaner module with subfiles was proposed by @thomwolf
(Can't find the discussion).

There's at least 3 parts than need to be explicited.

- The *glue code* that makes `pipeline` so powerful (loading up the right task for the right model with right task, basically completing all the wholes based on the call signature). That's `__init__.py`
- The main class `Pipeline` that mutualises a lot of the boilerplate. Thats `base.py`.
- All the specialized classes `NerPipeline`, `FeatureExtractionPipeline`, ... that's the other files.


All the tests remains strictly the same to ensure there's no breaking change in there.
The main issue with that PR is that now its a bit harder to check the various code flow.
Some is in the base, some is in a specialized file.

`TranslationPipeline`, `SummarizationPipeline` and `Text2TextPipeline` are in the same file `text2text_generation.py` as they seem to share quit a bit of code, maybe some cleanup and better code sharing is imaginable in a follow-up PR (https://github.com/Narsil/transformers/pull/1), at the very least modifications in one, probably should be duplicated in the others as they use the same underlying models (Seq2Seq).


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## Who can review?

@LysandreJik
@thomwolf



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Comments:
I think I fixed all of them. 

When moving everything around I felt more comfortable switching temporarily to absolute imports forgot to switch back.