text: Title: Simplify the command to download the data
  Body: ## 🌟 Feature Description
  <!-- A clear and concise description of the feature proposal -->
  
  Currently, user should use the following command to download the data.
  ![image](https://user-images.githubusercontent.com/465606/180944946-20cb4b7e-f46e-4ac6-8c03-621dcc261813.png)
  
  
  This requires the following steps.
  1. Install Qlib
  2. clone the source code of Qlib
  3.  Run the script to download the data
  
  This can be simplified by integrating the downloading script into a Qlib module. So users can download data with only two steps
  1. Install Qlib
  2. Run qlib module to download data like this `python -m qlib.XXX.get_data qlib_data --target_dir ~/.qlib/qlib_data/cn_data --region cn`
  