How does torch differentiate between batch and single values?


Here’s my neural network.

from torch import nn
from import DataLoader
class NeuralNetwork(nn.Module):
    def __init__(self, state_size, action_size):
        super(NeuralNetwork, self).__init__()
        self.state_size = state_size
        self.action_size = action_size
#         self.flatten = nn.Flatten()
        self.linear_relu_stack = nn.Sequential(
            nn.Linear(1, 30),
            nn.Linear(30, 30),
            nn.Linear(30, action_size)
    def forward(self, x):
        x = self.linear_relu_stack(x)
        return x

As you can imagine, it can compute tensor inputs of the shape torch.Size([1]). However, when I try to feed it batch data, for instance, shape torch.Size([10]) it throws the following error –

RuntimeError: mat1 and mat2 shapes cannot be multiplied (1x10 and 1x30)

For instance, this code works –

net = NeuralNetwork(10, 5)
x1 = torch.rand(1)
out = net(x1)

But this fails –

x2 = torch.rand(10)
out = net(x2)

>Solution :

You just need to change your inputs a little bit. Your code is expecting the net() to have a second dimension of 1, so it can multiply by nn.linear(1,30). The inner dimensions must match for matrix multiplication to occur. I.e. 10×1 * 1×30:

x2 = torch.rand(10,1)
out = net(x2)
torch.Size([10, 1])

Try this with any dimension x2 = torch.rand(100,1) etc. it still works.

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