pytorch中如何只让指定变量向后传播梯度?
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有以下公式,假如要让L对xvar求导:
(1)中,L对xvar的求导将同时计算out1部分和out2部分;
(2)中,L对xvar的求导只计算out2部分,因为out1的requires_grad=False;
(3)中,L对xvar的求导只计算out1部分,因为out2的requires_grad=False;
验证如下:
#!/usr/bin/env python2 # -*- coding: utf-8 -*- """ Created on Wed May 23 10:02:04 2018 @author: hy """ import torch from torch.autograd import Variable print("Pytorch version: {}".format(torch.__version__)) x=torch.Tensor([1]) xvar=Variable(x,requires_grad=True) y1=torch.Tensor([2]) y2=torch.Tensor([7]) y1var=Variable(y1) y2var=Variable(y2) #(1) print("For (1)") print("xvar requres_grad: {}".format(xvar.requires_grad)) print("y1var requres_grad: {}".format(y1var.requires_grad)) print("y2var requres_grad: {}".format(y2var.requires_grad)) out1 = xvar*y1var print("out1 requres_grad: {}".format(out1.requires_grad)) out2 = xvar*y2var print("out2 requres_grad: {}".format(out2.requires_grad)) L=torch.pow(out1-out2,2) L.backward() print("xvar.grad: {}".format(xvar.grad)) xvar.grad.data.zero_() #(2) print("For (2)") print("xvar requres_grad: {}".format(xvar.requires_grad)) print("y1var requres_grad: {}".format(y1var.requires_grad)) print("y2var requres_grad: {}".format(y2var.requires_grad)) out1 = xvar*y1var print("out1 requres_grad: {}".format(out1.requires_grad)) out2 = xvar*y2var print("out2 requres_grad: {}".format(out2.requires_grad)) out1 = out1.detach() print("after out1.detach(), out1 requres_grad: {}".format(out1.requires_grad)) L=torch.pow(out1-out2,2) L.backward() print("xvar.grad: {}".format(xvar.grad)) xvar.grad.data.zero_() #(3) print("For (3)") print("xvar requres_grad: {}".format(xvar.requires_grad)) print("y1var requres_grad: {}".format(y1var.requires_grad)) print("y2var requres_grad: {}".format(y2var.requires_grad)) out1 = xvar*y1var print("out1 requres_grad: {}".format(out1.requires_grad)) out2 = xvar*y2var print("out2 requres_grad: {}".format(out2.requires_grad)) #out1 = out1.detach() out2 = out2.detach() print("after out2.detach(), out2 requres_grad: {}".format(out1.requires_grad)) L=torch.pow(out1-out2,2) L.backward() print("xvar.grad: {}".format(xvar.grad)) xvar.grad.data.zero_()