text: Title: Concerns about the calculation of return
  Body: ## 🐛 Bug Description
  
  I found the calculation of return in backtest confusing. The return is defined on each day's basis [here](https://github.com/microsoft/qlib/blob/2ed806c8469904e441cac494fad2af104a4dbd1d/qlib/backtest/account.py#L260). The return on each day are summed up directly to obtain the annualized return [here](https://github.com/microsoft/qlib/blob/2ed806c8469904e441cac494fad2af104a4dbd1d/qlib/contrib/evaluate.py#L58). Shouldn't the total return be a product of (1 + r_i) minus 1? What's the problem here?
  
  ## To Reproduce
  
  Steps to reproduce the behavior:
  
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  ## Expected Behavior
  
  <!-- A clear and concise description of what you expected to happen. -->
  
  ## Screenshot
  
  <!-- A screenshot of the error message or anything shouldn't appear-->
  
  ## Environment
  
  **Note**: User could run `cd scripts && python collect_info.py all` under project directory to get system information
  and paste them here directly.
  
   - Qlib version: 2ed806c8469904e441cac494fad2af104a4dbd1d
   - Python version: 3.7
   - OS (`Windows`, `Linux`, `MacOS`): Linux
   - Commit number (optional, please provide it if you are using the dev version): 2ed806c8469904e441cac494fad2af104a4dbd1d
  
  ## Additional Notes
  
  <!-- Add any other information about the problem here. -->
  