Adding some quality of life for `pipeline` function.
Author: Narsil
Pull request: yes

# What does this PR do?


Adds two quality of life things on `pipeline` function.

- Make `task` optional `pipeline(model="gpt2")` should not be enough (will rely on the hub).
- Make `pipeline_class` overridable to ease override for users of the `pipeline` function.
  `pipeline(model="gpt2", pipeline_class=MyPipeline)` should enable you to use whatever
  pipeline you want.

Related to #14278

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Fixes # (issue)


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Comments:
I'll merge ahead.