minor fix (#313)

topi/python/topi/cuda/depthwise_conv2d_map.py

@@ -103,7 +103,7 @@ def schedule_depthwise_conv2d_map(op):
             if OP not in s.outputs:
                 s[OP].compute_inline()
             for tensor in OP.input_tensors:
-                if str(tensor.op.input_tensors) != str([]):
+                if tensor.op.input_tensors:
                     traverse(tensor.op)
         # schedule depthwise_conv2d
         if OP.tag == 'depthwise_conv2d':