Loss.backward() raises error 'grad can be implicitly created only for scalar outputs'


loss.backward() do not go through while training and throws an error when on multiple GPUs using torch.nn.DataParallel

grad can be implicitly created only for scalar outputs

But, the same thing trains fine when I give only deviced_ids=[0] to torch.nn.DataParallel.
Is there something I am missing here?


While running on two gpus, the loss function returns a vector of 2 loss values. If I run the backward only on the first element of the vector it goes fine.

How can I make the backward function work with vector containing two or more loss values?



when you do loss.backward(), it is a shortcut for loss.backward(torch.Tensor([1])). This in only valid if loss is a tensor containing a single element.
DataParallel returns to you the partial loss that was computed on each gpu, so you usually want to do loss.backward(torch.Tensor([1, 1])) or loss.sum().backward(). Both will have the exact same behaviour.


If I want to get average loss of each sample, and every single element in loss has been averaged on batch, should I use loss.mean().backward()?

If all the batch are the same size, it will work.


I got it. Thank you : )

When I try loss.mean.backward() or loss.sum.backward() I am getting this warning? UserWarning: Was asked to gather along dimension 0, but all input tensors were scalars; will instead unsqueeze and return a vector. warnings.warn('Was asked to gather along dimension 0, but all ’

How do I suppress this one?

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Let’s continue the discussion in the topic you’ve created and have a look at @SimonW’s answer.

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Why is backward limited in this way? Is it an oversight or some important reason?


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The gradient is computed as a vector jacobian product. So the size of the vector has to match the size of a dimension jacobian (which is the size of the output).


Sure, the number of grads needs to equal the number of variables.

What I meant was it seems weird that the “backward” function is defined as on 1 variable unless otherwise stated, even though it is implemented on a vector with > 1 variables.

e.g. if I have vec, a 2 element tensor of 2 variables, and call vec.backward() it won’t work, but if vec is a 1 element tensor it will. I can’t see an obvious reason why backward should be default limited to 1 variable (unless explicitly told otherwise), especially seeing as it is a method of the variable.

Is there a reason for this limitation?

You can call it with more elements, it’s just that you have to specify the grads yourself.

can you write a example?

For example, if you want to compute the gradient for the sum of the elements in x. You can do either:
x.sum().backward() or x.backward(torch.ones_like(x)).