[dist.cp] Introduce LoadPlanner and SavePlanner extensibility API. (#83419)
The planners come with default implementations in default_planner.py.
The default planners expose their core functionality as separate functions
to make it easy for other checkpoint implementations to use this functionality.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83419
Approved by: https://github.com/wanchaol
