Fix issue with setting of param_groups/defaults/state for the DPOptimizer wrapper (meta-pytorch#660)

Summary:
Pull Request resolved: meta-pytorch#660

Fix for github issue # [649](meta-pytorch#649)

**Background**: DPOptimizer is a wrapper for the original non-DP Optimizer selected by the user. `param_groups`, `state`, `defaults` are parameter of DPOPtimizer that store all parameters related to the learning algorithm, including privacy-related parameters.

**Issue**: Previously, DPOptimizer passed `param_groups`, `state`, `defaults` simply by reference.  Thus another object can update param_groups for the DPOptimizer, while neglecting to update such parameters for the original Optimizer. The issue is reflected e.g., in the LR (learning rate scheduler) where the learning rate looks as if its being updated for the DPOptimizer, but it is not actually updated for the original Optimizer (the one that matters).

**Fix**: In this fix, we use the property decorator to ensure that the 3 parameters remain the same between DPOptimizer and Optimizer.

Differential Revision: D60453849

Author: iden-kalemaj

opacus/optimizers/optimizer.py

@@ -15,6 +15,7 @@
 from __future__ import annotations
 
 import logging
+from collections import defaultdict
 from typing import Callable, List, Optional, Union
 
 import torch
@@ -376,6 +377,50 @@ def accumulated_iterations(self) -> int:
             )
         return vals[0]
 
+    @property
+    def param_groups(self) -> List[dict]:
+        """
+        Returns a list containing a dictionary of all parameters managed by the optimizer.
+        """
+        return self.original_optimizer.param_groups
+
+    @param_groups.setter
+    def param_groups(self, param_groups: List[dict]):
+        """
+        Updates the param_groups of the optimizer, where param_groups is a list containg a dictionary
+        of all parameters mangaged by the optimizer.
+        """
+        self.original_optimizer.param_groups = param_groups
+    
+
+    @property 
+    def state(self) -> defaultdict:
+        """
+        Returns a dictionary holding current optimization state.
+        """
+        return self.original_optimizer.state
+    
+    @state.setter
+    def state(self, state: defaultdict):
+        """
+        Updates the state of the optimizer, where state is a dictionary holding current optimization state.
+        """
+        self.original_optimizer.state = state
+    
+    @property
+    def defaults(self) -> dict:
+        """
+        Returns a dictionary containing default values for optimization.
+        """
+        return self.original_optimizer.defaults
+    
+    @defaults.setter
+    def defaults(self, defaults: dict):
+        """
+        Updates the defaults of the optimizer, where defaults is a dictionary containing default values for optimization.
+        """
+        self.original_optimizer.defaults = defaults
+
     def attach_step_hook(self, fn: Callable[[DPOptimizer], None]):
         """
         Attaches a hook to be executed after gradient clipping/noising, but before the