Simplify observers declaration with functools.partial (#26492)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26492
Previous definition of observers was quite clumsy - with things like `default_observer()()`. This PR strips a way a lot of craft and allows to pass just class names directly. In order to override default arguments either `functools.partial` can be used or convenient wrapper `MyObserver.with_args(x=1)` is provided.
Also rename `QConfig_dynamic` to `QConfigDynamic` because it violates the naming convention.
Test Plan: Imported from OSS
Differential Revision: D17521265
Pulled By: dzhulgakov
fbshipit-source-id: ba9df19b368641acf4093c43df9990796284fd9e
