PPO examples in pl_examples/domain_templates/ not working

[MouseHu]

When running ppo examples in pl_examples/domain_templates/， I met the following error：

AttributeError: 'PPOLightning' object has no attribute '_param_requires_grad_state'

After some debugging， I found this error was due to this function:

def optimizer_step(self, *args, **kwargs):
    for _ in range(self.nb_optim_iters):
        super().optimizer_step(*args, **kwargs)

Comment out this function makes the example running normally.
I am using pl 1.2.0 and I think it is either a bug in this example or API upgrades making this example incompatible with the newer pl. Any suggestions to fix this problem?

[MouseHu]

@tchaton Could you please have a look at this?