Changes to compile with 3.13 (#126033)

This is mainly:
- Fix refcount access macro
- Hide all the Dynamo code that needs update as usual
- Add _PyWeakref_ClearRef as an extern provided by CPython. Including the pycore header that defines it would require raw c include shenanigans that I don't think are worth it.
This allows to build both with regular and nogil version of cpython. Both

Note that this requires the 3.13 branch at least past [d3094744d40de2deefbda9b1996d5029c9ebf0b0](d3094744d4) which we need for mimalloc include and weakref function being exposed.

debug-only issues in pybind11 with PyMem_MALLOC vs PyObject_MALLOC being should be synced either by updating pybind or cpython. @colesbury I can send a PR to ifdef the proper use in pybind if you think that this is the best solution here?

Pull Request resolved: https://github.com/pytorch/pytorch/pull/126033
Approved by: https://github.com/colesbury

torch/csrc/autograd/python_variable.cpp

@@ -1931,7 +1931,7 @@ void THPVariable_subclass_dealloc(PyObject* self) {
   if (type->tp_del) {
     PyObject_GC_Track(self);
     type->tp_del(self);
-    if (self->ob_refcnt > 0) {
+    if (Py_REFCNT(self) > 0) {
       /* Resurrected */
       return;
     }