该函数接受三个参数，分别是:
tf.random_normal(shape, mean=0.0, stddev=1.0, dtype=tf.float32, seed=None, name=None)
输出的数据呢，是服从标准正态分布的，shape是数据的形状(m*n的矩阵)，dtype是其类型。
这里的用法和np.random.normal(loc=0.0, scale=1.0, size=None)略像，
loc：Mean (“centre”) of the distribution
scale:Standard deviation (spread or “width”) of the distribution.
size : Output shape. If the given shape is, e.g., (m, n, k), then m * n * k samples are drawn. If size is None (default), a single value is returned if loc and scale are both scalars. Otherwise, np.broadcast(loc, scale).size samples are drawn.