Qualcomm AI Engine Direct - support multi-context, dlbc (#2450)

Summary:
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- refactor compiler spec for more backends
- support HTP features of multi-context, dlbc
- test cases for multi-context

Pull Request resolved: https://github.com/pytorch/executorch/pull/2450

Reviewed By: kirklandsign

Differential Revision: D54932332

Pulled By: cccclai

fbshipit-source-id: 5570897c1ab3833fc1c0b45d0331469aece4cf31