self.sub_cl2 uses an inplace ReLU as the first layer, which will (as the name suggests) apply the relu inplace on the input tensor, which will thus apply it on x_sub_classifier1.
You could set the inplace argument to False or alternatively clone x_sub_classifier1 before passing it to self.sub_cl2.
PS: Unrelated to your current issue, but you should use nn.ModuleList instead of Python lists, as the former approach will make sure to properly register all parameters.