[JIT] Add static method support for TorchBind (#51177)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51177
**Summary**
This commit adds support for static methods to TorchBind. Just like
pybind, the API for declaring a static method is `def_static(...)`. A
static method must be called on the class directly, and can be called
both in Python as well as TorchScript.
Support for static methods is implemented in a manner similar to that of
instance methods. Registered static functions are wrapped in a layer of
unboxing logic, their schemas are inferred using templates and
metaprogramming, and they are added to the `ClassType` object
corresponding to the TorchBind class on which they are registered.
ScriptClass has been extended to support a `__getattr__` function so
that static methods of TorchBind classes can be invoked in Python. The
implementation of `__getattr__` returns `ScriptClassFunctionPtr`, a
version of `StrongFunctionPtr` without a compilation unit (since the
functions of a TorchBind class live inside the TorchBind registry).
Within TorchScript, TorchBind static functions are desugared in
`PythonClassValue::attr` by looking them up on the class type of the
`PythonClassValue` instance.
**Test Plan**
This commit adds a unit test that tests a simple static method on a
TorchBind class.
Test Plan: Imported from OSS
Reviewed By: pbelevich
Differential Revision: D26356942
Pulled By: SplitInfinity
fbshipit-source-id: 1b6a9bc2e5f3e22071ad78e331a0201fbbf7ab30
