dont memory plan buffers (#2415)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2415

constant buffers should not be memory planned.

bypass-github-export-checks

Reviewed By: ydwu4

Differential Revision: D54877663

fbshipit-source-id: 9d48b044d49161e19dfdb9f98a2997a00d381346