[autograd.Function] Kill the extension feature flag (#92026)

This PR removes the autograd.Function extension feature flag. This was
previously used for development of the functorch <> autograd.Function
interaction.

It's been in master for long enough with the feature flag defaulting to
True, so it's time to remove it.

Test Plan:
- existing tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/92026
Approved by: https://github.com/soulitzer

torch/csrc/autograd/python_function.cpp

@@ -912,8 +912,7 @@ PyObject* THPFunction_apply(PyObject* cls, PyObject* inputs) {
   // autograd.Function may optionally contain a setup_context staticmethod.
   // In this case, autograd.Function.forward does NOT accept a ctx object.
   bool has_separate_setup_context_fn =
-      (isAutogradFunctionExtensionEnabled() &&
-       PyObject_HasAttrString(cls, "setup_context"));
+      PyObject_HasAttrString(cls, "setup_context");
 
   auto num_args = PyTuple_GET_SIZE(inputs);