fix attribute name mismatch in state_dict() and load_state_dict()

apex/contrib/optimizers/fp16_optimizer.py

@@ -196,7 +196,7 @@ class FP16_Optimizer(object):
             state_dict['scale_factor'] = self.scale_factor
             state_dict['scale_window'] = self.scale_window
         state_dict['optimizer_state_dict'] = self.optimizer.state_dict()
-        state_dict['fp32_groups_flat'] = self.fp32_groups_flat
+        state_dict['fp32_groups'] = self.fp32_groups
         return state_dict
 
     def load_state_dict(self, state_dict):
@@ -238,5 +238,5 @@ class FP16_Optimizer(object):
         # the current optimizer instance.  In our case, as long as the current FP16_Optimizer has been
         # constructed in the same way as the one whose state_dict we are loading, the same master params
         # are guaranteed to exist, so we can just copy_() from the saved master params.
-        for current, saved in zip(self.fp32_groups_flat, state_dict['fp32_groups_flat']):
+        for current, saved in zip(self.fp32_groups, state_dict['fp32_groups']):
             current.data.copy_(saved.data)