Sorry for the delay here @JingL, I was able to run things with
pip install torchvision==0.9.1
Update to nightly torch:
pip install --upgrade --pre --find-links https://download.pytorch.org/whl/nightly/cpu/torch_nightly.html torch==1.10.0.dev20210715+cpu
# ./speed_benchmark_torch --pthreadpool_size=1 --model=mobilenetv2-none-nnapi.ptl  --use_bundled_input=0 --warmup=5 --iter=200                                                                           
Starting benchmark.
Running warmup runs.
Main runs.
Main run finished. Microseconds per iter: 137647. Iters per second: 7.26494

Changes in converter script:

nnapi_model._save_for_lite_interpreter(“mobilenetv2-{}-nnapi.ptl”.format(quantize_mode))

Please let me know if this works for you