Qualcomm AI Engine Direct Backend (#490)

Summary:
Hi there,

This commit adds QNN backend. Please check setup.md and README under `backends/qnn` for more details.

We add pybind dependency at the top level, because we guess you might need it in the near future. If it's not the case, please feel free to tell us. We'll move it under `backends/qnn`.

Below is some SoC model mapping:

- `SM8550` -> `Snapdragon 8 Gen 2`
- `SM8475` -> `Snapdragon 8 Gen 1+`
- `SM8450` -> `Snapdragon 8 Gen 1`

Please feel free to contact us if you find any problem.

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Pull Request resolved: https://github.com/pytorch/executorch/pull/490

Reviewed By: cccclai

Differential Revision: D49686980

Pulled By: kimishpatel

fbshipit-source-id: 5398137bd015a056de80f55a921f4d718df08a24

Co-authored-by: shewu-quic <quic_shewu@quicinc.com>
Co-authored-by: chunit-quic <quic_chunit@quicinc.com>
Co-authored-by: chiwwang <quic_chiwwang@quicinc.com>
Co-authored-by: harshs-qti <quic_harshs@quicinc.com>