how to divide a map into many little cells in ros 2?

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I'm a beginner. I'm using ros 2 - foxy and turtlebot3 burger. I follow the turtlebot3 manual and I get a map (map.yaml and map.pgm) after run slam simualation. I want to implement a complete coverage path planner. So first, i have to divide my map into many little cells (grid map) and visualize this grid map in rviz2. I have read grid_map package document https://github.com/ANYbotics/grid_map. From the grid_map package document, I found the "grid_map_loader" package, but I don't know how to use that package to load my map.