make norm compatible with pytorch version <= 1.0.1

apex/optimizers/fp16_optimizer.py

@@ -116,7 +116,11 @@ class FP16_Optimizer(object):
         """
         # TODO: Not most efficient with copy to cpu and sync
         # only support 2-norm now
-        norm = float(torch.norm(fp16_grads_flat, 2.0, dtype=torch.float32))
+        # for torch version <= 1.0.1, torch.norm with dtype will fail and fall back to cast
+        try:
+            norm = float(torch.norm(fp16_grads_flat, 2.0, dtype=torch.float32))
+        except TypeError as err:
+            norm = float(torch.norm(fp16_grads_flat.float(), 2.0))
         if norm == float('inf') or norm == -float('inf') or norm != norm:
             return -1
         else: