Autograd for complex-valued neural networks

I have a doubt for autograd for complex-valued neural networks(Autograd mechanics — PyTorch 1.11.0 documentation).It seems that autograd works when differentiating complex-valued tensors. But does it also work for layers in a neural network?
So lets say I define a F.Linear layer, can we define the weights to be complex-valued and will the updates to this layer follow complex-valued backpropogation? Also, can the same thing be done for nn.Conv2d layer?

Hi Anirudh!

Yes, autograd performs complex backpropagation through (most, if not
all) pytorch layers and with respect to those layers’ parameters.

Consider this Linear example:

>>> import torch
>>> torch.__version__
>>> _ = torch.manual_seed (2022)
>>> lin = torch.nn.Linear (2, 2, bias = False, dtype = torch.complex64)
>>> lin.weight
Parameter containing:
tensor([[-0.1474+0.5967j,  0.3660-0.1681j],
        [-0.6700-0.1989j,  0.4149+0.3975j]], requires_grad=True)
>>> t = torch.randn (5, 2, dtype = torch.complex64)
>>> t
tensor([[-0.5672+0.5518j, -0.8078+0.3621j],
        [ 0.0075+0.4234j, -0.4115-0.4382j],
        [-1.5592+0.4851j,  0.6728-0.1796j],
        [ 0.4976+0.4027j, -0.9824-0.7075j],
        [-0.1651-0.3127j, -0.6328-0.4980j]])
>>> lin (t).norm().backward()
>>> lin.weight.grad
tensor([[-0.3748+1.0385j,  0.6620-0.4970j],
        [-1.2673-0.4408j,  0.8804+0.7391j]])

Note, the implementation of complex gradients has built into it, in some
sense, the notion that you are calculating the gradient of a real-valued
loss function.

If you calculate the gradient of a complex “loss” function, you might not
get the results you expect (depending upon your expectations).

Pytorch’s support for complex manipulations is still a work in progress, but
most (if not all) of the basics have been implemented and work correctly.


K. Frank

Hi K.Frank,
Thanks for replying.I was trying to do the same for convolutional layer.
The following code is a normal convolution operation.

m = torch.nn.Conv2d(16, 33, (3, 3))
input = torch.randn(20, 16, 50, 50)
output = m(input)

But when I do it for complex-convolution,

m = torch.nn.Conv2d(16, 33, (3, 3),dtype=torch.complex64)
input = torch.randn(20, 16, 50, 50,dtype=torch.complex64)
output = m(input)

It gives me the following error,

RuntimeError: “slow_conv2d_cpu” not implemented for ‘ComplexFloat’.

Hi Anirudh!

You’re right, Conv2d does not yet have complex support in the latest
stable release (1.11.0) – that’s my fault for not double-checking.

(As I said, pytorch’s complex support is still a work in progress.)

However, Conv2d does have complex support in the latest nightly build
(a 1.12 version):

>>> import torch
>>> torch.__version__
>>> _ = torch.manual_seed (2022)
>>> conv = torch.nn.Conv2d (1, 1, 3, dtype = torch.complex64)
>>> conv (torch.randn (1, 1, 3, 3, dtype = torch.complex64))
tensor([[[[0.1691-0.2932j]]]], grad_fn=<AddBackward0>)


K. Frank