You have to pass the default parameters to the optimizer, too.
This should work:
optimizer = torch.optim.SGD([
	{'params': model.act_max},
        {'params': model.conv1.parameters(), 'weight_decay': 0.001},
	{'params': model.conv2.parameters(), 'weight_decay': 0.002},
	{'params': model.linear.parameters(), 'weight_decay': 0.003}], lr=0.01, momentum=0.9, nesterov=True)