Fix to enable freezing params

apex/parallel/distributed.py

@@ -136,7 +136,7 @@ class DistributedDataParallel(Module):
                     
             torch.cuda.current_stream().wait_stream(self.reduction_stream)
 
-        for param_i, param in enumerate(list(self.module.parameters())):
+        for param_i, param in enumerate([p for p in self.module.parameters() if p.requires_grad]):
             def wrapper(param_i):
                 
                 def allreduce_hook(*unused):
@@ -203,7 +203,7 @@ class DistributedDataParallel(Module):
         
     def forward(self, *inputs, **kwargs):
 
-        param_list = [param for param in list(self.module.parameters()) if param.requires_grad]
+        param_list = [param for param in self.module.parameters() if param.requires_grad]
 
 
         #Force needs_refresh to True if there are shared params