Your current hook function doesn’t do anything with the activation, so you could add e.g. a print statement:
def hook(module, input, output):
    print(output.shape)

model = nn.Conv2d(1, 1, 3)

with torch.no_grad():
    model.register_forward_hook(hook)
    embed = model(torch.randn(1, 1, 4, 4))