Qualcomm AI Engine Direct - enable loading context binary directly (#4163)

Summary:
- add utilities for loading context binary generated from qnn tools
- align env variable naming with qnn
- fix bug in online prepare and extend coverage to support bitwise quatization
- llama7b e2e example from qualcomm ai_hub
- minor fixes for syle & typo

Pull Request resolved: https://github.com/pytorch/executorch/pull/4163

Reviewed By: swolchok, kirklandsign

Differential Revision: D59737140

Pulled By: cccclai

fbshipit-source-id: 16e98d7f5da7204a2d04258fd75dabd8aa1eaa7d