Don't launch for empty sets

apex/optimizers/fused_sgd.py

@@ -154,11 +154,12 @@ class FusedSGD(Optimizer):
                                [fp32_grads, fp32_params, fp32_momentums]]
 
             for launch_set, first_run in zip(launch_sets, first_runs):
+                assert len(launch_set[0]) == len(launch_set[1])
+                assert len(launch_set[0]) == len(launch_set[2])
+                if len(launch_set[0]) > 0:
                     multi_tensor_applier(
                         self.multi_tensor_sgd,
                         self._dummy_overflow_buf,
-                    # Note: Need to do this as list comprehensions otherwise
-                    # things don't seem to update properly.
                         launch_set,
                         weight_decay,
                         momentum,