Accumulate gradients in a subset of parameters only

Is it possible to perform backpropagation and accumulate gradients in just a subset of the leaf variables involved in the computational graph? My solution now is to call autograd.grad and then copy the result in the .grad tensors of those parameters. Is there a way to do this directly?

import torch.nn as nn
import torch.nn.functional as F
import torch.autograd as autograd

model1 = nn.Sequential(nn.Linear(10, 10), nn.ReLU(), nn.Linear(10, 10))
model2 = nn.Sequential(nn.Linear(10, 10), nn.ReLU(), nn.Linear(10, 10))
noise = torch.randn(5, 10)
target1, target2 = (torch.randint(10, (5,)).long() for _ in range(2))

data = model1(noise)
output = model2(data)

loss1 = F.cross_entropy(output, target1)  # we care about model1's params
loss2 = F.cross_entropy(output, target2)  # we care about model2's params


# -------------------------------------------------------------------------
# do something such that model2 has grads of loss2 while
# model1's grads remain untouched.

grads = autograd.grad(loss2, model2.parameters())
for param, grad in zip(model2.parameters(), grads):

I’m not sure, how exactly your code works, but do you think you could play with the requires_grad attribute of your models?
E.g. in case you want to use loss1 to calculate the gradients for model1, but not model2, you could just disable gradients for model2:

for param in model2.parameters():
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