NameError: name 'Grid' is not defined"

Hi! I have a problem and sincerely hope to get your help!
Problem description: I wrote a python program myself to implement some functions of the project, but I encountered a problem in the first step.
In grids.py, line 66, grid: Grid, I didn't see the definition or import of Grid. In the end, I implemented the def velocity_trajectory_to_xarray() function of xarray_utils.py of the data submodule in the python file I wrote and reported an error " NameError: name 'Grid' is not defined", what should I do about this?