DOC add AR model example

[MatthewSZhang]

Checklist

• Used a personal fork to propose changes
• A reference to a related issue: DOC add examples for Auto-Regressive models #122
• A description of the changes:

• add an CO2 forecasting example to demonstrate how to use NARX to build an AR model
• make NARX accept None input
• fix some bugs about gradient computation, when using AR models and y_init or y contains nan

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (84b2f83) to head (084c9de).
⚠️ Report is 75 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #129   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         9    +3     
  Lines          603       647   +44     
=========================================
+ Hits           603       647   +44     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.