Difference between "detach()" and "with torch.nograd()" in PyTorch?

Question

I know about two ways to exclude elements of a computation from the gradient calculation backward

Method 1: using with torch.no_grad()

with torch.no_grad():     y = reward + gamma * torch.max(net.forward(x)) loss = criterion(net.forward(torch.from_numpy(o)), y) loss.backward(); 

Method 2: using .detach()

y = reward + gamma * torch.max(net.forward(x)) loss = criterion(net.forward(torch.from_numpy(o)), y.detach()) loss.backward(); 

Is there a difference between these two? Are there benefits/downsides to either?

Answer 1

tensor.detach() creates a tensor that shares storage with tensor that does not require grad. It detaches the output from the computational graph. So no gradient will be backpropagated along this variable.

The wrapper with torch.no_grad() temporarily set all the requires_grad flag to false. torch.no_grad says that no operation should build the graph.

The difference is that one refers to only a given variable on which it is called. The other affects all operations taking place within the with statement. Also, torch.no_grad will use less memory because it knows from the beginning that no gradients are needed so it doesn’t need to keep intermediary results.

Learn more about the differences between these along with examples from here.