perf: fix missing noexcepts on minpybind in functorch (#94135)

Noticed this performance bug in functorch. We got a pretty big perf in pybind11 improvement by explicitly marking at noexcept, see https://quuxplusone.github.io/blog/2022/08/26/vector-pessimization/

Pull Request resolved: https://github.com/pytorch/pytorch/pull/94135
Approved by: https://github.com/ezyang

functorch/csrc/dim/minpybind.h

@@ -59,8 +59,7 @@ struct vector_args;
 struct handle {
     handle(PyObject* ptr)
     : ptr_(ptr) {}
-    handle()
-    : ptr_(nullptr) {}
+    handle() = default;
 
 
     PyObject* ptr() const {
@@ -90,7 +89,7 @@ struct handle {
     }
 
 protected:
-    PyObject * ptr_;
+    PyObject* ptr_ = nullptr;
 };
 
 
@@ -107,7 +106,7 @@ struct hdl : public handle {
     }
     hdl(T* ptr)
     : hdl((PyObject*) ptr) {}
-    hdl(obj<T> o)
+    hdl(const obj<T>& o)
     : hdl(o.ptr()) {}
 private:
     hdl(handle h) : handle(h) {}
@@ -119,14 +118,14 @@ struct object : public handle {
     : handle(other.ptr_) {
         Py_XINCREF(ptr_);
     }
-    object(object&& other)
+    object(object&& other) noexcept
     : handle(other.ptr_) {
         other.ptr_ = nullptr;
     }
     object& operator=(const object& other) {
         return *this = object(other);
     }
-    object& operator=(object&& other) {
+    object& operator=(object&& other) noexcept {
         PyObject* tmp = ptr_;
         ptr_ = other.ptr_;
         other.ptr_ = tmp;
@@ -165,14 +164,14 @@ struct obj : public object {
     : object(other.ptr_) {
         Py_XINCREF(ptr_);
     }
-    obj(obj&& other)
+    obj(obj&& other) noexcept
     : object(other.ptr_) {
         other.ptr_ = nullptr;
     }
     obj& operator=(const obj& other) {
         return *this = obj(other);
     }
-    obj& operator=(obj&& other) {
+    obj& operator=(obj&& other) noexcept {
         PyObject* tmp = ptr_;
         ptr_ = other.ptr_;
         other.ptr_ = tmp;
@@ -503,7 +502,7 @@ struct dict_view : public handle {
         return PyDict_Check(h.ptr());
     }
     bool next(Py_ssize_t* pos, py::handle* key, py::handle* value) {
-        PyObject *k, *v;
+        PyObject *k = nullptr, *v = nullptr;
         auto r = PyDict_Next(ptr(), pos, &k, &v);
         *key = k;
         *value = v;