Fixing the "translation", "translation_XX_to_YY" pipelines.
Author: Narsil
Pull request: yes

# What does this PR do

Actually make the "translation", "translation_XX_to_YY" task behave correctly.

Background:
- Currently "translation_cn_to_ar" does not work. (only 3 pairs are
supported)
- Some models, contain in their config the correct values for the (src,
tgt) pair they can translate. It's usually just one pair, and we can
infer it automatically from the `model.config.task_specific_params`. If
it's not defined we can still probably load the TranslationPipeline
nevertheless.

Proposed fix:
- A simplified version of what could become more general which is
a `parametrized` task. "translation" + (src, tgt) in this instance
it what we need in the general case. The way we go about it for now
is simply parsing "translation_XX_to_YY". If cases of parametrized task arise
we should preferably go in something closer to what `datasets` propose
which is having a secondary argument `task_options`? that will be close
to what that task requires.
- Should be backward compatible in all cases for instance
`pipeline(task="translation_en_to_de") should work out of the box.
- Should provide a warning when a specific translation pair has been
selected on behalf of the user using
`model.config.task_specific_params`.



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Comments:
But If 1 models implies 1 task, how should we cope with models that are able to do multiple things (like `t5-base`) ? 

I think `datasets` does it correctly in that it does not make any choice on your behalf, but instead raises an Exception with your available choices.

> But If 1 models implies 1 task, how should we cope with models that are able to do multiple things (like `t5-base`) ?
> 
> I think `datasets` does it correctly in that it does not make any choice on your behalf, but instead raises an Exception with your available choices.

I think T5Base should default to a `ForConditionalPipeline` (which is more or less the `Text2TextPipeline` we have right now). Then the user could either provide a pipeline config that makes sure "summarization" or "translation" params are used for the pipeline. Note: In the end of the day all Seq2Seq pipelines are exactly the same -> they are all based on `.generate()` and they only differ on which params (`max_length`, `prefix`, ...) are used. Or/And we create a very shallow "alias" pipeline named `class TranslationPipeline(ConditionalGenerationPipeline)` that only overwrites the config params similar to what we do here: https://github.com/huggingface/transformers/blob/901e9b8eda2fe88af717f960ddc05cac1803679b/src/transformers/pipelines.py#L568 right now. 

Maybe @mfuntowicz can also give a bit more context on the Pipeline v2 vision here.