[SR][easy] Accessors for value array offsets (#69755)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69755
Per swolchok's suggestion on D32609915 (https://github.com/pytorch/pytorch/commit/1c43b1602ce3c77a1a0c64868a5358ea79c730ba). Hide the value offset indices behind accessors to provide more flexibility if we ever decide to change the layout of the values array.
Test Plan: `buck test caffe2/benchmarks/static_runtime:static_runtime_cpptest`
Reviewed By: hlu1
Differential Revision: D32838145
fbshipit-source-id: cf805c077672de4c2fded9b41da01eca6d84b388
