【PyTorch】深度学习实战之反向传播
PyTorch实现反向传播
- 反向传播主要体现在l.backward()，调用该方法后w.grad由None更新为Tensor类型，且w.grad.data用于后序w.data的更新
- l.backward()会把计算图中所有需要梯度的地方都会求出来，然后把梯度都存在对应待求的参数中，最终计算图被释放
实现代码：
import torch
x_data = [1.0, 2.0, 3.0]
y_data = [2.0, 4.0, 6.0]
w = torch.tensor([1.0])
w.requires_grad = True
def forward(x):
return x * w; # w是一个Tensor
def loss(x, y):
y_pred = forward(x)
return (y_pred - y) ** 2
print("predict (before training))", 4, forward(4).item())
for epoch in range(100):
for x, y in zip(x_data, y_data):
l = loss(x, y) # l是一个张量，tensor主要是建立在计算图，计算损失
l.backward()
print('\tgrad:', x, y, w.grad.item())
w.data = w.data - 0.01 * w.grad.data # 权重更新时，注意grad也是一个Tensor
w.grad.data.zero_() # 更新完之后要记得清零
print("progress:", epoch, l.item())
print("predict (after training)", 4, forward(4).item())
训练结果如下：
predict (before training)) 4 4.0
grad: 1.0 2.0 -2.0
grad: 2.0 4.0 -7.840000152587891
grad: 3.0 6.0 -16.228801727294922
progress: 0 7.315943717956543
grad: 1.0 2.0 -1.478623867034912
grad: 2.0 4.0 -5.796205520629883
grad: 3.0 6.0 -11.998146057128906
progress: 1 3.9987640380859375
grad: 1.0 2.0 -1.0931644439697266
grad: 2.0 4.0 -4.285204887390137
grad: 3.0 6.0 -8.870372772216797
progress: 2 2.1856532096862793
grad: 1.0 2.0 -0.8081896305084229
grad: 2.0 4.0 -3.1681032180786133
grad: 3.0 6.0 -6.557973861694336
progress: 3 1.1946394443511963
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progress: 8 0.0582793727517128
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progress: 94 9.094947017729282e-13
grad: 1.0 2.0 -7.152557373046875e-07
grad: 2.0 4.0 -2.86102294921875e-06
grad: 3.0 6.0 -5.7220458984375e-06
progress: 95 9.094947017729282e-13
grad: 1.0 2.0 -7.152557373046875e-07
grad: 2.0 4.0 -2.86102294921875e-06
grad: 3.0 6.0 -5.7220458984375e-06
progress: 96 9.094947017729282e-13
grad: 1.0 2.0 -7.152557373046875e-07
grad: 2.0 4.0 -2.86102294921875e-06
grad: 3.0 6.0 -5.7220458984375e-06
progress: 97 9.094947017729282e-13
grad: 1.0 2.0 -7.152557373046875e-07
grad: 2.0 4.0 -2.86102294921875e-06
grad: 3.0 6.0 -5.7220458984375e-06
progress: 98 9.094947017729282e-13
grad: 1.0 2.0 -7.152557373046875e-07
grad: 2.0 4.0 -2.86102294921875e-06
grad: 3.0 6.0 -5.7220458984375e-06
progress: 99 9.094947017729282e-13
predict (after training) 4 7.999998569488525
| 如果没办法缓解对未来的焦虑，那就先做好眼前的事情吧。大雾的清晨是无法看清远方的，但至少脚下的每一步是清晰的。总会有大雾散开的时刻，在那之前，你不能囿于原地，一步都不走。 |
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