no attribute 'if_off_policy'

Hi all,

I just install and run the main.py for the first time. I bump on to the `Arguments(agent_class=agent)` error then I change `agent_class` to `agent`.
After this I get another error as the following:
```
File "/home/jason/FinRL/FinRL-Library/finrl/main.py", line 68, in main
train(
File "/home/jason/FinRL/FinRL-Library/finrl/train.py", line 59, in train
model = agent.get_model(model_name, model_kwargs=erl_params)
File "/home/jason/FinRL/FinRL-Library/finrl/agents/elegantrl/models.py", line 63, in get_model
model = Arguments(agent=agent, env=env)
File "/home/jason/FinRL/FinRL-Library/venv/lib/python3.9/site-packages/elegantrl-0.3.3-py3.9.egg/elegantrl/train/config.py", line 127, in __init__
self.if_off_policy = agent.if_off_policy # agent is on-policy or off-policy
AttributeError: module 'elegantrl.agents.AgentPPO' has no attribute 'if_off_policy'
```
I notice that `self.if_off_policy` is assigned one line after this error line. How can I solve this problem

Thanks