Fixing concatenation error for fp16 ditributed training

distributed.py

@@ -140,7 +140,7 @@ def apply_gradient_allreduce(module):
                 buckets = {}
                 for param in module.parameters():
                     if param.requires_grad and param.grad is not None:
-                        tp = type(param.data)
+                        tp = param.data.dtype
                         if tp not in buckets:
                             buckets[tp] = []
                         buckets[tp].append(param)