MindSpore 自动微分

代码原地址：

https://www.mindspore.cn/tutorial/zh-CN/r1.2/autograd.html

MindSpore计算一阶导数方法  mindspore.ops.GradOperation (get_all=False, get_by_list=False, sens_param=False)，其中get_all为False时，只会对第一个输入求导，为True时，会对所有输入求导；

get_by_list为False时，不会对权重求导，为True时，会对权重求导；

sens_param对网络的输出值做缩放以改变最终梯度。

get_all : 决定着是否根据输出对输入进行求导。

get_by_list : 决定着是否对神经网络内的参数权重求导。

sens_param : 对网络的输出进行乘积运算后再求导。（通过对网络的输出值进行缩放后再进行求导）

网络的前向传播：

import numpy as np
import mindspore.nn as nn
import mindspore.ops as ops
from mindspore import Tensor
from mindspore import ParameterTuple, Parameter
from mindspore import dtype as mstype

class Net(nn.Cell):
    def __init__(self):
        super(Net, self).__init__()
        self.matmul = ops.MatMul()
        self.z = Parameter(Tensor(np.array([1.0, 1.0, 1.0], np.float32)), name='z')

    def construct(self, x, y):
        x = x * self.z
        out = self.matmul(x, y)
        return out

model = Net()

for m in model.parameters_and_names():
    print(m)

x = Tensor([[0.8, 0.6, 0.2], [1.8, 1.3, 1.1]], dtype=mstype.float32)
y = Tensor([[0.11, 3.3, 1.1], [1.1, 0.2, 1.4], [1.1, 2.2, 0.3]], dtype=mstype.float32)
result = model(x, y)
print(result)

n_x = np.array([[0.8, 0.6, 0.2], [1.8, 1.3, 1.1]], dtype=np.float32)
n_y = np.array([[0.11, 3.3, 1.1], [1.1, 0.2, 1.4], [1.1, 2.2, 0.3]], dtype=np.float32)
result = model(x, y)
print(result)

=======================================================

反向传播：

import numpy as np
import mindspore.nn as nn
import mindspore.ops as ops
from mindspore import Tensor
from mindspore import ParameterTuple, Parameter
from mindspore import dtype as mstype

class Net(nn.Cell):
    def __init__(self):
        super(Net, self).__init__()
        self.matmul = ops.MatMul()
        self.z = Parameter(Tensor(np.array([1.0, 1.0, 1.0], np.float32)), name='z')

    def construct(self, x, y):
        x = x * self.z
        out = self.matmul(x, y)
        return out

class GradNetWrtX(nn.Cell):
    def __init__(self, net):
        super(GradNetWrtX, self).__init__()
        self.net = net
        self.grad_op = ops.GradOperation()

    def construct(self, x, y):
        gradient_function = self.grad_op(self.net)
        return gradient_function(x, y)

x = Tensor([[0.8, 0.6, 0.2], [1.8, 1.3, 1.1]], dtype=mstype.float32)
y = Tensor([[0.11, 3.3, 1.1], [1.1, 0.2, 1.4], [1.1, 2.2, 0.3]], dtype=mstype.float32)
output = GradNetWrtX(Net())(x, y)
print(output)

因为， ops.GradOperation()，

mindspore.ops.GradOperation (get_all=False, get_by_list=False, sens_param=False)

get_all=False,  表示只对第一个输入向量求导，也就是对x 求梯度， 结果得到x的梯度：

如果想对所有的输入求梯度，  ops.GradOperation(get_all=True)

代码：

import numpy as np
import mindspore.nn as nn
import mindspore.ops as ops
from mindspore import Tensor
from mindspore import ParameterTuple, Parameter
from mindspore import dtype as mstype

class Net(nn.Cell):
    def __init__(self):
        super(Net, self).__init__()
        self.matmul = ops.MatMul()
        self.z = Parameter(Tensor(np.array([1.0, 1.0, 1.0], np.float32)), name='z')

    def construct(self, x, y):
        x = x * self.z
        out = self.matmul(x, y)
        return out

class GradNetWrtX(nn.Cell):
    def __init__(self, net):
        super(GradNetWrtX, self).__init__()
        self.net = net
        self.grad_op = ops.GradOperation(get_all=True)

    def construct(self, x, y):
        gradient_function = self.grad_op(self.net)
        return gradient_function(x, y)

x = Tensor([[0.8, 0.6, 0.2], [1.8, 1.3, 1.1]], dtype=mstype.float32)
y = Tensor([[0.11, 3.3, 1.1], [1.1, 0.2, 1.4], [1.1, 2.2, 0.3]], dtype=mstype.float32)
output = GradNetWrtX(Net())(x, y)
print(len(output))
print('='*30)
print(output[0])
print('='*30)
print(output[1])

对网络的所有输入求导，即，对 x ,y  求梯度。

对权重求一阶导

对权重求一阶导数其实与前面相比有两个地方要更改：

1.   求导函数要写明对权重求导，即传入参数 get_by_list=True

即，

self.grad_op = ops.GradOperation(get_by_list=True)

2.  具体求导时要传入具体待求导的参数（即，权重）：

self.params = ParameterTuple(net.trainable_params())

gradient_function = self.grad_op(self.net, self.params)

细节：

需要知道的一点是如果我们设置了对权重求梯度，则默认不会再对输入求梯度：

代码：

import numpy as np
import mindspore.nn as nn
import mindspore.ops as ops
from mindspore import Tensor
from mindspore import ParameterTuple, Parameter
from mindspore import dtype as mstype

class Net(nn.Cell):
    def __init__(self):
        super(Net, self).__init__()
        self.matmul = ops.MatMul()
        self.z = Parameter(Tensor(np.array([1.0, 1.0, 1.0], np.float32)), name='z')

    def construct(self, x, y):
        x = x * self.z
        out = self.matmul(x, y)
        return out

class GradNetWrtX(nn.Cell):
    def __init__(self, net):
        super(GradNetWrtX, self).__init__()
        self.net = net
        self.params = ParameterTuple(net.trainable_params())
        self.grad_op = ops.GradOperation(get_by_list=True)

    def construct(self, x, y):
        gradient_function = self.grad_op(self.net, self.params)
        return gradient_function(x, y)

model = Net()

x = Tensor([[0.8, 0.6, 0.2], [1.8, 1.3, 1.1]], dtype=mstype.float32)
y = Tensor([[0.11, 3.3, 1.1], [1.1, 0.2, 1.4], [1.1, 2.2, 0.3]], dtype=mstype.float32)
output = GradNetWrtX(model)(x, y)
print(len(output))
print('='*30)
print(output[0])
print('='*30)

如果对网络内部权重求梯度同时也想对输入求梯度，只能显示的设置 get_all=True,

即，

self.grad_op = ops.GradOperation(get_all=True, get_by_list=True)

此时输出的梯度为 元组 ，    tuple（所有输入的梯度， 内部权重梯度）

也就是说这种情况返回的是一个两个元素的元组，元组中第一个元素是所有输入的梯度，第二个是所有内部权重的梯度。

代码：

import numpy as np
import mindspore.nn as nn
import mindspore.ops as ops
from mindspore import Tensor
from mindspore import ParameterTuple, Parameter
from mindspore import dtype as mstype

class Net(nn.Cell):
    def __init__(self):
        super(Net, self).__init__()
        self.matmul = ops.MatMul()
        self.z = Parameter(Tensor(np.array([1.0, 1.0, 1.0], np.float32)), name='z')

    def construct(self, x, y):
        x = x * self.z
        out = self.matmul(x, y)
        return out

class GradNetWrtX(nn.Cell):
    def __init__(self, net):
        super(GradNetWrtX, self).__init__()
        self.net = net
        self.params = ParameterTuple(net.trainable_params())
        self.grad_op = ops.GradOperation(get_all=True, get_by_list=True)

    def construct(self, x, y):
        gradient_function = self.grad_op(self.net, self.params)
        return gradient_function(x, y)

model = Net()

x = Tensor([[0.8, 0.6, 0.2], [1.8, 1.3, 1.1]], dtype=mstype.float32)
y = Tensor([[0.11, 3.3, 1.1], [1.1, 0.2, 1.4], [1.1, 2.2, 0.3]], dtype=mstype.float32)
output = GradNetWrtX(model)(x, y)
print(len(output))
print('='*30)
print(output[0])
print('='*30)
print(output[1])

运行结果：

[WARNING] ME(6838:140435163703104,MainProcess):2021-07-06-16:56:49.250.395 [mindspore/_check_version.py:109] Cuda version file version.txt is not found, please confirm that the correct cuda version has been installed, you can refer to the installation guidelines: https://www.mindspore.cn/install
2
==============================
(Tensor(shape=[2, 3], dtype=Float32, value=
[[ 4.50999975e+00,  2.70000005e+00,  3.60000014e+00],
 [ 4.50999975e+00,  2.70000005e+00,  3.60000014e+00]]), Tensor(shape=[3, 3], dtype=Float32, value=
[[ 2.59999990e+00,  2.59999990e+00,  2.59999990e+00],
 [ 1.89999998e+00,  1.89999998e+00,  1.89999998e+00],
 [ 1.30000007e+00,  1.30000007e+00,  1.30000007e+00]]))
==============================
(Tensor(shape=[3], dtype=Float32, value= [ 1.17259989e+01,  5.13000011e+00,  4.68000031e+00]),)

进程已结束，退出代码为 0