Description : build network

def build_model(self, dr_rate=0, path=None):
        """
        Description : build network
        """
        #set network
        self.net = LipNet(dr_rate)
        self.net.hybridize()
        self.net.initialize(ctx=self.ctx)

        if path is not None:
            self.load_model(path)

        #set optimizer
        self.loss_fn = gluon.loss.CTCLoss()
        self.trainer = gluon.Trainer(self.net.collect_params(), \
                                     optimizer='SGD')