[ez] Document for add_var_to_val (#121850)
Summary: Add doc for ShapeEnv.add_var_to_val
Test Plan: doc only change
Reviewed By: izaitsevfb
Differential Revision: D54872335
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121850
Approved by: https://github.com/izaitsevfb
