Qualcomm AI Engine Direct - support online prepare (#1692)

Summary:
Summary
- online graph building mechanism for incoming backends (GPU/DSP)
- introduce flatbuffer to help IR serialization
- change compiler spec / test spec accordingly

Pull Request resolved: https://github.com/pytorch/executorch/pull/1692

Reviewed By: kirklandsign

Differential Revision: D53947011

Pulled By: cccclai

fbshipit-source-id: 523f4245b2cd36b0e7abde94f41cfee26252b15b