Update kernel key version comment (#1349)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1349

Kernel key version tracks the kernel key format for bc/fc.

If the format is changed later, we can prepend with v1, vn, so that we know how to parse the new format, as well as the old format

Reviewed By: guangy10

Differential Revision: D51835693

fbshipit-source-id: 257ce8f7fc104d0d71f5dd878b47ebecc1814f8e