Add getPyInterpreter() API (pytorch#62659)

Summary:
Pull Request resolved: pytorch#62659

It turns out that it is occasionally useful to be able to access the
PyInterpreter object from other Python bindings (see next diff in the
stack).  Make it publicly available.

Signed-off-by: Edward Z. Yang <[email protected]>

Test Plan: Imported from OSS

Reviewed By: albanD

Differential Revision: D30074926

Pulled By: ezyang

fbshipit-source-id: 2f745ab7c7a672ed7215231fdf9eef6af9705511

Author: ezyang

torch/csrc/autograd/python_variable.cpp

@@ -107,6 +107,10 @@ PyInterpreterHolder self_interpreter;
 
 } // anonymous namespace
 
+c10::impl::PyInterpreter* getPyInterpreter() {
+  return self_interpreter.get();
+}
+
 namespace py = pybind11;
 
 PyObject *THPVariableClass = nullptr;

torch/csrc/autograd/python_variable.h

@@ -51,3 +51,5 @@ inline const at::Tensor& THPVariable_Unpack(THPVariable* var) {
 inline const at::Tensor& THPVariable_Unpack(PyObject* obj) {
   return THPVariable_Unpack(reinterpret_cast<THPVariable*>(obj));
 }
+
+THP_API c10::impl::PyInterpreter* getPyInterpreter();