Add qnn recipe

[cccclai]

Add QNN recipe and test it with dit model

We have the following features added

1. Add qnn.py recipe
2. Test it with dit model

Install executorch and qnn from source by running

./install_executorch.sh
./backends/qualcomm/scripts/build.sh

The following commands are failing

pytest -k test_dit_image_classification_qnn -m run_slow 
RUN_SLOW=1 pytest -k test_deit_image_classification -m run_slow 
pytest -k test_bert_fill_mask_qnn -m run_slow 

The nightly build should work once executorch viable/strict get updated. Then we can use nightly wheel package to install qnn instead of manually install

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[jackzhxng]

@cccclai Let's try an ET pin bump?