caffe2: refactor context to allow being typed (#48340)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48340
This changes the context managed classes from using a decorator to define them to using inheritance. Inheritance allows the python static type checking to work correctly.
```
context.define_context()
class Bar(object): ...
context.define_context(allow_default=True)
class Foo(object): ...
```
becomes
```
class Foo(context.Managed): ...
class Bar(context.DefaultManaged): ...
```
Behavior differences:
* arg_name has been removed since it's not used anywhere
* classes need to call `super()` in `__enter__/__exit__` methods if they override (none do)
This also defines a context.pyi file to add types for python3. python2 support should not be affected
Test Plan:
ci
buck test //caffe2/caffe2/python:context_test //caffe2/caffe2/python:checkpoint_test
Reviewed By: dongyuzheng
Differential Revision: D25133469
fbshipit-source-id: 16368bf723eeb6ce3308d6827f5ac5e955b4e29a
