text: Title: Refine Qlib's code style reported by pylint and flake8
  Body: ## 🌟 Feature Description
  <!-- A clear and concise description of the feature proposal -->
  
  ## Motivation
  `Qlib` has integrated pylint and flake8 to automatically detect the problem of coding style 
  
  But [some errors](https://github.com/microsoft/qlib/blob/main/.github/workflows/test.yml#L39) are not fixed due to the limited bandwidth of the maintainers.
  It will be very helpful if the community would like to contribute to refine them :)
  
  ![image](https://user-images.githubusercontent.com/465606/159872365-9c64c9ac-20b8-4f06-a7ae-990c2aada13e.png)
  
  
  ## Alternatives
  
  <!-- A short description of any alternative solutions or features you've considered. -->
  
  ## Additional Notes
  
  <!-- Add any other context or screenshots about the feature request here. -->