Update on "[Gradient Compression] Refactor CommHookInterface and Pyth…

…onCommHook."

1. Let CommHookInterface::processFuture be a non-virtual method, which can be shared by both Python and C++ implementations.
2. Merge 1-line PythonCommHook constructor into the header for simplicity.
3. Rename processFuture method as parseFromHookResult for readability.
4. Simplify the comments.

Original PR issue: C++ DDP Communication Hook #46348

Differential Revision: [D24374282](https://our.internmc.facebook.com/intern/diff/D24374282/)

[ghstack-poisoned]

torch/csrc/distributed/c10d/comm.h

@@ -54,7 +54,7 @@ class TORCH_API CommHookInterface {
 class TORCH_PYTHON_API PythonCommHook : public CommHookInterface {
  public:
   PythonCommHook(py::object state, py::object hook)
-      : state_(std::move(state)), hook_(std::move(hook)){};
+      : state_(std::move(state)), hook_(std::move(hook)) {}
 
   ~PythonCommHook() override {
     py::gil_scoped_acquire ag;