

TITLE
Clear up confusing translation pipeline task naming

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# 🚀 Feature request
Hello! 
I am using the translation pipeline, and I noticed that even though I have to specify the language when I create the pipeline, the passed model overwrites that. So pipeline created as
`nlp = pipeline('translation_en_to_de', 'Helsinki-NLP/opus-mt-en-jap')`
would translate english to japanese, in contrary to the task name. Is this the intended way of translating other languages, will it change in the future?

Would it be possible to just add a single 'translation' task for pipelines, which would then resolve the languages based on the model (which it seems to do anyway now) ?

## Motivation
It would clear up the current confusion, and make the `pipeline` function singature less prone to change.
It could also possibly reduce code duplication in https://github.com/huggingface/transformers/blob/master/src/transformers/pipelines.py
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## My contribution
I'd love to help with a PR, though I'm confused: The `SUPPORTED_TASKS` dictionary in pipelines.py contains exactly the same entries for each translation pipeline, even the default model is the same, yet the specific pipelines actually translate to different languages 🤔 