Refactor autograd package to separate Python dependencies. (#662)

The core autograd Variable, Function, and Engine no longer depend on the
Python API. This let's us implement functions in C++. In the future, we
can also multithread engine and release the GIL for most of the
non-Python backwards.

torch/csrc/utils.cpp

@@ -577,11 +577,4 @@ void THPPointer<THPGenerator>::free() {
     Py_DECREF(ptr);
 }
 
-template<>
-void THPPointer<PyObject>::free() {
-  if (ptr)
-    Py_DECREF(ptr);
-}
-
 template class THPPointer<THPGenerator>;
-template class THPPointer<PyObject>;