text: Title: ENH: implement `np.tri`
  Body: ### Is your feature request related to a problem? Please describe
  `np.tri` is a wildly used function. It is basicly a combination of `np.ones` and `np.tril` and should be easy to implement.
  
  Check this out for how to implement a mars operand: https://docs.pymars.org/en/latest/development/operand.html
  
  And the impl of `ones` may be a good example: https://github.com/xprobe-inc/xorbits/blob/main/python/xorbits/_mars/tensor/datasource/ones.py
  