Instantiate the model

def instantiate(self, **extra_args):
        """ Instantiate the model """
        input_block = self.input_block.instantiate()
        policy_backbone = self.policy_backbone.instantiate(**extra_args)
        value_backbone = self.value_backbone.instantiate(**extra_args)

        return StochasticPolicyModelSeparate(input_block, policy_backbone, value_backbone, extra_args['action_space'])