change build mode to release (#1219)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1219

It seems by default it maybe building with release mode. This results in much
slower flatbuffer serialization.

Before:
MV3: 37s
SAM (local changes): ~7m

After:
MV3: 31s
SAM (local changes): ~4m

Reviewed By: kirklandsign

Differential Revision: D51310857

fbshipit-source-id: 5bb934ba5f069fd627351b93548aed2bf81e9a0c