Created by: zhiqiu

As the title, this PR adds unary operator __neg__ for VarBase in dygraph mode, and for Variable in static graph mode.

Example,

import paddle.fluid as fluid
import numpy as np

x_np = np.ones([1, 2], np.float32)
with fluid.dygraph.guard():
    x = fluid.dygraph.to_variable(x_np)
    print(x.numpy())  # [[1, 1]]
    print(-x.numpy())  # [[-1, -1]]