Created by: zhiqiu

This PR patches math methods, like +, -, *, /, for VarBase, and the patched methods will auto-generated op functions, as described in PR21569.

Sample code.

import paddle.fluid as fluid
import numpy as np

a_np = np.asarray([3])
b_np = np.asarray([1])
place = fluid.CUDAPlace(0)
with fluid.dygraph.guard(place):
    a = fluid.dygraph.to_variable(a_np)
    b = fluid.dygraph.to_variable(b_np)
    res = a * b
    print(res.numpy())  # [3]