numpy: np.newaxis

用于增加array的dimension
比如：
1D 会变成 2D，2D会变成3D……

x1 = np.array([1, 2, 3])
np.shape(x1)  # (3,)

x2 = x1[:, np.newaxis]
np.shape(x2) # (3, 1)
x3 = x1[np.newaxis, :]
np.shape(x3) # (1, 3)

未完