Created by: QiJune

Please refer to #5197 I add another unit test case to test pow operator in GPU, the test case is

class TestPow1(OpTest):
    def setUp(self):
        self.op_type = "pow"
        self.inputs = {
            'X': np.random.uniform(-2, 2, [11, 17]).astype("float32")
        }
        self.attrs = {'factor': 2.0}
        self.outputs = {'Y': np.power(self.inputs['X'], 2)}

    def test_check_output(self):
        self.check_output()

    def test_check_grad(self):
        self.check_grad(['X'], 'Y', max_relative_error=0.02)

And it will get nan in GPU:

[22:36:10] :	 [Step 1/1] 145/248 Test #149: test_activation_op ..........................***Failed    3.36 sec
[22:36:10] :	 [Step 1/1] ................../paddle/python/paddle/v2/framework/tests/op_test.py:328: RuntimeWarning: invalid value encountered in greater
[22:36:10] :	 [Step 1/1]   offset = np.argmax(diff_mat > max_relative_error)
[22:36:10] :	 [Step 1/1] FF............................
[22:36:10] :	 [Step 1/1] ======================================================================
[22:36:10] :	 [Step 1/1] FAIL: test_check_grad (__main__.TestPow1)
[22:36:10] :	 [Step 1/1] ----------------------------------------------------------------------
[22:36:10] :	 [Step 1/1] Traceback (most recent call last):
[22:36:10] :	 [Step 1/1]   File "test_activation_op.py", line 329, in test_check_grad
[22:36:10] :	 [Step 1/1]     self.check_grad(['X'], 'Y', max_relative_error=0.02)
[22:36:10] :	 [Step 1/1]   File "/paddle/python/paddle/v2/framework/tests/op_test.py", line 382, in check_grad
[22:36:10] :	 [Step 1/1]     "Gradient Check On %s" % str(gpu_place))
[22:36:10] :	 [Step 1/1]   File "/paddle/python/paddle/v2/framework/tests/op_test.py", line 334, in __assert_is_close
[22:36:10] :	 [Step 1/1]     self.assertLessEqual(max_diff, max_relative_error, err_msg())
[22:36:10] :	 [Step 1/1] AssertionError: Gradient Check On GPUPlace(0) Variable X max gradient diff nan over limit 0.020000, the first error element is 0, 0.008404, 0.008405