Based on the error message, it seems the input to the first conv layer is given as a 2-dimensional tensor.
This is probably your embedding.
An nn.Conv2d layer expects a 4-dimensional input with the shapes [batch_size, channels, height, width].
Based on the setup of your first conv layer and your drawing, I assume you would like to use a single channel with a spatial size of [8, 8].
If that’s the case, try to unsqueeze the batch and channel dimension using:
# x.shape == [8, 8]
x = x.unsqueeze(0).unsqueeze(1)
# x.shape == [1, 1, 8, 8]

However, could you also post the first code snippet including the embedding, as apparently your batch dimension went missing, which could be another (hidden) error in your code.