Some Lookahead cleanup and fixes

Author: rwightman

timm/optim/lookahead.py

@@ -13,37 +13,40 @@ def __init__(self, base_optimizer, alpha=0.5, k=6):
             raise ValueError(f'Invalid slow update rate: {alpha}')
         if not 1 <= k:
             raise ValueError(f'Invalid lookahead steps: {k}')
-        self.alpha = alpha
-        self.k = k
+        defaults = dict(lookahead_alpha=alpha, lookahead_k=k, lookahead_step=0)
         self.base_optimizer = base_optimizer
         self.param_groups = self.base_optimizer.param_groups
         self.defaults = base_optimizer.defaults
+        self.defaults.update(defaults)
         self.state = defaultdict(dict)
-        for group in self.param_groups:
-            group["step_counter"] = 0
+        # manually add our defaults to the param groups
+        for name, default in defaults.items():
+            for group in self.param_groups:
+                group.setdefault(name, default)
 
-    def update_slow_weights(self, group):
+    def update_slow(self, group):
         for fast_p in group["params"]:
             if fast_p.grad is None:
                 continue
             param_state = self.state[fast_p]
-            if "slow_buffer" not in param_state:
-                param_state["slow_buffer"] = torch.empty_like(fast_p.data)
-                param_state["slow_buffer"].copy_(fast_p.data)
-            slow = param_state["slow_buffer"]
-            slow.add_(self.alpha, fast_p.data - slow)
+            if 'slow_buffer' not in param_state:
+                param_state['slow_buffer'] = torch.empty_like(fast_p.data)
+                param_state['slow_buffer'].copy_(fast_p.data)
+            slow = param_state['slow_buffer']
+            slow.add_(group['lookahead_alpha'], fast_p.data - slow)
             fast_p.data.copy_(slow)
 
     def sync_lookahead(self):
         for group in self.param_groups:
-            self.update_slow_weights(group)
+            self.update_slow(group)
 
     def step(self, closure=None):
+        #assert id(self.param_groups) == id(self.base_optimizer.param_groups)
         loss = self.base_optimizer.step(closure)
         for group in self.param_groups:
-            group['step_counter'] += 1
-            if group['step_counter'] % self.k == 0:
-                self.update_slow_weights(group)
+            group['lookahead_step'] += 1
+            if group['lookahead_step'] % group['lookahead_k'] == 0:
+                self.update_slow(group)
         return loss
 
     def state_dict(self):
@@ -52,37 +55,36 @@ def state_dict(self):
             (id(k) if isinstance(k, torch.Tensor) else k): v
             for k, v in self.state.items()
         }
-        fast_state = fast_state_dict["state"]
-        param_groups = fast_state_dict["param_groups"]
+        fast_state = fast_state_dict['state']
+        param_groups = fast_state_dict['param_groups']
         return {
-            "state": fast_state,
-            "slow_state": slow_state,
-            "param_groups": param_groups,
+            'state': fast_state,
+            'slow_state': slow_state,
+            'param_groups': param_groups,
         }
 
     def load_state_dict(self, state_dict):
+        fast_state_dict = {
+            'state': state_dict['state'],
+            'param_groups': state_dict['param_groups'],
+        }
+        self.base_optimizer.load_state_dict(fast_state_dict)
+
+        # We want to restore the slow state, but share param_groups reference
+        # with base_optimizer. This is a bit redundant but least code
+        slow_state_new = False
         if 'slow_state' not in state_dict:
-            print('Loading state_dict from optimizer without Lookahead applied')
+            print('Loading state_dict from optimizer without Lookahead applied.')
             state_dict['slow_state'] = defaultdict(dict)
+            slow_state_new = True
         slow_state_dict = {
-            "state": state_dict["slow_state"],
-            "param_groups": state_dict["param_groups"],
-        }
-        fast_state_dict = {
-            "state": state_dict["state"],
-            "param_groups": state_dict["param_groups"],
+            'state': state_dict['slow_state'],
+            'param_groups': state_dict['param_groups'],  # this is pointless but saves code
         }
         super(Lookahead, self).load_state_dict(slow_state_dict)
-        self.base_optimizer.load_state_dict(fast_state_dict)
-
-    def add_param_group(self, param_group):
-        r"""Add a param group to the :class:`Optimizer` s `param_groups`.
-        This can be useful when fine tuning a pre-trained network as frozen
-        layers can be made trainable and added to the :class:`Optimizer` as
-        training progresses.
-        Args:
-            param_group (dict): Specifies what Tensors should be optimized along
-            with group specific optimization options.
-        """
-        param_group['step_counter'] = 0
-        self.base_optimizer.add_param_group(param_group)
+        self.param_groups = self.base_optimizer.param_groups  # make both ref same container
+        if slow_state_new:
+            # reapply defaults to catch missing lookahead specific ones
+            for name, default in self.defaults.items():
+                for group in self.param_groups:
+                    group.setdefault(name, default)