Discrepancy in train/run.py produces TypeError: '>=' not supported between instances of 'list' and 'int' error

Hi,

In my quest to understand how the code works, I noticed in the _run_ function of the _PipeWorker_ class, this line:

`gpu_id = args.learner_gpus`

But _args.learner_gpus_ seems to be a list of IDs, where gpu_id a single ID.

And indeed when running the [code ](https://elegantrl.readthedocs.io/en/latest/tutorial/elegantrl-podracer.html), I get the error

```
File "/workspace/elegantrl/agents/AgentBase.py", line 45, in __init__
self.device = torch.device(f"cuda:{gpu_id}" if (torch.cuda.is_available() and (gpu_id >= 0)) else "cpu")
TypeError: '>=' not supported between instances of 'list' and 'int'
```
Thanks for your help