[Refactor] Seperate the constant part from config.py.

[ChiahungTai]

🌟 Feature Description

Right now the REGION CONST is a global variable. Maybe we can refactor it to ENUM.
Also maybe we should seperate a constant.py for the constants.

Motivation

1. Application scenario
2. Related works (Papers, Github repos etc.):
3. Any other relevant and important information:

1. REGION CONST is suitbale for ENUM in natural.
2. Seperate to constant.py can avoid circular import in the future.

Alternatives

Additional Notes

[you-n-g]

@ChiahungTai
I strongly agree with you!

[ChiahungTai]

@you-n-g
After more thoughts, I think change REGION_CONST as an enum might not a good idea.
Beacuse once it is enum, user is hard to add new REGION_CONST without change the config/constant.py.
But move the constant as seperate file is still good idea.
I would like to change the tile to only Seperate constants from config.py.