import error about `build_env` and solution

[Yonv1943]

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Search the funtion build_env in ElegantRL. There are:

21 code results in AI4Finance-Foundation/ElegantRL

The from elegantrl.envs.Gym import build_env could be changed to from elegantrl.train.config import build_env

The function build_env has been moved from elegantrl/envs/Gym.py to elegantrl/train/config.py

https://github.com/AI4Finance-Foundation/ElegantRL/blob/51fce54cc6e5263b413059d74c0991070869396f/elegantrl/train/config.py#L204-L219