fix pylint

Author: pengwa

tests/test_backend.py

@@ -848,7 +848,7 @@ def test_split(self):
     def test_split_with_more_outputs(self):
         x_val = np.linspace(1.0, 5 * 30.0, 5 * 30).astype(np.float32).reshape(5, 30)
         x0 = tf.placeholder(tf.float32, x_val.shape, name=_TFINPUT)
-        x_, y, z = tf.split(x0, [4, 15, 11], 1, name="split_test")
+        _, _, _ = tf.split(x0, [4, 15, 11], 1, name="split_test")
         self._run_test_case(["split_test:0", "split_test:1", "split_test:2"], {_INPUT: x_val})
 
     def test_reducesum(self):