功能：通过equation进行矩阵乘法。
输入：equation：乘法算法定义。
# 矩阵乘
>>> einsum('ij,jk->ik', m0, m1)  # output[i,k] = sum_j m0[i,j] * m1[j, k]
# 点乘
>>> einsum('i,i->', u, v)  # output = sum_i u[i]*v[i]
# 向量乘
>>> einsum('i,j->ij', u, v)  # output[i,j] = u[i]*v[j]
# 转置
>>> einsum('ij->ji', m)  # output[j,i] = m[i,j]
# 批量矩阵乘
>>> einsum('aij,ajk->aik', s, t)  # out[a,i,k] = sum_j s[a,i,j] * t[a, j, k]
例：
a = tf.constant([[1,2],[3,4]])
b = tf.constant([[5,6],[7,8]])
z=tf.einsum('ij,jk->ik',a,b)

z==>[[19 22]
     [43 50]]