[fx] Don't use __module__ to test if a function is bound from C++
The new `test_public_bindings.py` test means `__module__` will be set
correctly in future, even for functions bound from C++. Instead, just
test directly that the function is of the `BuiltinFunctionType` which
only passes for functions exported with the CPython API.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/75896
Approved by: https://github.com/albanD
