class Controller(nn.Module):
    r"""
    This class is the basic class for all controller implementations.
    """
    def __init__(self):
        pass

    def get_control(self, parameters, state, ref_state, feed_forward_quantity):
        r"""
        Args:
            parameters: controller parameters, e.g. kp, ki, kd in PID controllers.
            state:  current system state of dynamic systems.
            ref_state:  the reference state of systems to reach.
            feed_forward_quantity: system plant input generated by other feed forward
                controllers.
        """
        pass

    def reset(self):
        r"""
        This method is used to reset the internal state.
        For example, integrated error in PID controller
        """
        pass

class ControllerParametersTuner():
    def __init__(self, learning_rate):
        self.learning_rate = learning_rate
   
    r"""
    Args:
            dynamic_system (pypose.module.dynamics):
            initial_state (Tensor):
            ref_states ():
            controller (pypose.module.controller): Linear or nonlinear controller to control
            the dynamic system
            parameters (Tensor): Controller parameters
            parameters_tuning_set (Tensor): This set gives the minimum and the maximum
                value the parameters can reach
            tau: time interval in system
            states_to_tune (Tensor): choose which state needs to be considered in the
                loss function, usually only position is chosen.
            func_get_state_error (function): function has two inputs: system state and
                ref state. The function needs to be provided by users considering system
                state and ref state are not always in the same formation.
    """
    def tune(self, dynamic_system, initial_state, ref_states, 
              controller, parameters, parameters_tuning_set, tau, states_to_tune, func_get_state_error):
        ...