RuntimeError: mat1 and mat2 shapes cannot be multiplied (5400x64 and 5400x64)

Question

I'm working on an image classification network and got a problem with the right values of inputs and outputs in the forward() function. I don't have an idea to solve this, because they seem the same to me. The error comes from this line: x = F.relu(self.fc1(x)), but I can't figure it out.

Can anyone please help me with this problem?

That's my code:

class Net(nn.Module):
    def __init__(self):
        super(Net, self).__init__()
        self.conv1 = nn.Conv2d(3, 8, kernel_size=2)
        self.conv2 = nn.Conv2d(8, 12, kernel_size=2)
        self.conv3 = nn.Conv2d(12, 18, kernel_size=2)
        self.conv4 = nn.Conv2d(18, 24, kernel_size=2)
        self.fc1 = nn.Linear(5400, 64)
        self.fc2 = nn.Linear(64, 2)

    def forward(self, x):
        print(f'1. {x.size()}')
        x = self.conv1(x)
        x = F.max_pool2d(x, 2)
        x = F.relu(x)
        print(f'2. {x.size()}')
        x = self.conv2(x)
        x = F.max_pool2d(x, 2)
        x = F.relu(x)
        print(f'3. {x.size()}')
        x = self.conv3(x)
        x = F.max_pool2d(x, 2)
        x = F.relu(x)
        print(f'4. {x.size()}')
        x = self.conv4(x)
        x = F.max_pool2d(x, 2)
        x = F.relu(x)
        print(f'5. {x.size()}')
        x = x.view(-1, x.size(0)) 
        print(f'6. {x.size()}')
        x = F.relu(self.fc1(x))
        print(f'7. {x.size()}')
        x = self.fc2(x)
        print(f'8. {x.size()}')
        
        return torch.sigmoid(x)

That's the print output:

1. torch.Size([64, 3, 256, 256])
2. torch.Size([64, 8, 127, 127])
3. torch.Size([64, 12, 63, 63])
4. torch.Size([64, 18, 31, 31])
5. torch.Size([64, 24, 15, 15])
6. torch.Size([5400, 64])

Answer 1

I think changing

x = x.view(-1, x.size(0))

to

x = x.view([-1, 5400], x.size(0))

Will solve your problem, You see that in print 6:

6. torch.Size([5400, 64])

the batch size 64 is in the 1 axes and not in the 0 axes. The fully connected layer expects an input of size 5400 therefore changing this will likely solve since you do not know that batch size but you know that the input to the fully-connected is 5400.