[WIP] hsigmoid op

[Yancey0623]

Fixed #4971

TODO

1. Fix test backward failed

61: ERROR: test_check_grad (__main__.TestHSigmoidOp)
61: ----------------------------------------------------------------------
61: Traceback (most recent call last):
61:   File "test_hsigmoid_op.py", line 35, in test_check_grad
61:     no_grad_set=set(['Label']))
61:   File "/paddle/python/paddle/v2/fluid/tests/op_test.py", line 374, in check_grad
61:     in_place=in_place) for input_to_check in inputs_to_check
61:   File "/paddle/python/paddle/v2/fluid/tests/op_test.py", line 141, in get_numeric_gradient
61:     __set_elem__(tensor_to_check, i, x_pos)
61:   File "/paddle/python/paddle/v2/fluid/tests/op_test.py", line 127, in __set_elem__
61:     tensor.set_int64_element(i, e)
61: TypeError: set_int64_element(): incompatible function arguments. The following argument types are supported:
61:     1. (self: paddle.v2.fluid.core.Tensor, arg0: int, arg1: int) -> None
61:
61: Invoked with: <paddle.v2.fluid.core.LoDTensor object at 0x7f33ec279510>, 0, 1.005

2. Implement hsigmoid with Python code in unit test.
3. Fix backward failed.