In fact, the issue was solved by performing a deepcopy of my original model, as shown below.
    old_model = MyModelClass.to(device)
    state_dict = torch.load(model_state_dict_path)
    old_model.load_state_dict(state_dict)

    model = copy.deepcopy(old_model)

Anyways, I don’t know yet why these steps were necessary in order to make my model change its weights and be able to learn