QNN: wav2letter doesn't export

🐛 Describe the bug

python -m examples.qualcomm.scripts.wav2letter -b cmake-out -m SM8450 --compile_only -p ./states_fused.pth

fails to export. See stacktrace:

QNN_SDK_ROOT=/home/guangyang/fbsource/third-party/qualcomm/qnn/qnn-2.26
Traceback (most recent call last):
  File "/data/users/guangyang/executorch/examples/qualcomm/scripts/wav2letter.py", line 220, in <module>
    main(args)
  File "/data/users/guangyang/executorch/examples/qualcomm/scripts/wav2letter.py", line 131, in main
    inputs, targets, input_list = get_dataset(
  File "/data/users/guangyang/executorch/examples/qualcomm/scripts/wav2letter.py", line 69, in get_dataset
    for wave, label in data_loader:
  File "/home/guangyang/.conda/envs/executorch/lib/python3.10/site-packages/torch/utils/data/dataloader.py", line 708, in __next__
    data = self._next_data()
  File "/home/guangyang/.conda/envs/executorch/lib/python3.10/site-packages/torch/utils/data/dataloader.py", line 764, in _next_data
    data = self._dataset_fetcher.fetch(index)  # may raise StopIteration
  File "/home/guangyang/.conda/envs/executorch/lib/python3.10/site-packages/torch/utils/data/_utils/fetch.py", line 52, in fetch
    data = [self.dataset[idx] for idx in possibly_batched_index]
  File "/home/guangyang/.conda/envs/executorch/lib/python3.10/site-packages/torch/utils/data/_utils/fetch.py", line 52, in <listcomp>
    data = [self.dataset[idx] for idx in possibly_batched_index]
  File "/home/guangyang/.conda/envs/executorch/lib/python3.10/site-packages/torchaudio/datasets/librispeech.py", line 170, in __getitem__
    waveform = _load_waveform(self._archive, metadata[0], metadata[1])
  File "/home/guangyang/.conda/envs/executorch/lib/python3.10/site-packages/torchaudio/datasets/utils.py", line 51, in _load_waveform
    waveform, sample_rate = torchaudio.load(path)
  File "/home/guangyang/.conda/envs/executorch/lib/python3.10/site-packages/torchaudio/_backend/utils.py", line 204, in load
    backend = dispatcher(uri, format, backend)
  File "/home/guangyang/.conda/envs/executorch/lib/python3.10/site-packages/torchaudio/_backend/utils.py", line 116, in dispatcher
    raise RuntimeError(f"Couldn't find appropriate backend to handle uri {uri} and format {format}.")
RuntimeError: Couldn't find appropriate backend to handle uri ./wav2letter/LibriSpeech/test-clean/2094/142345/2094-142345-0042.flac and format None.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/guangyang/.conda/envs/executorch/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/home/guangyang/.conda/envs/executorch/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/data/users/guangyang/executorch/examples/qualcomm/scripts/wav2letter.py", line 226, in <module>
    raise Exception(e)
Exception: Couldn't find appropriate backend to handle uri ./wav2letter/LibriSpeech/test-clean/2094/142345/2094-142345-0042.flac and format None.

where states_fused.pth is downloaded from https://github.com/nipponjo/wav2letter-ctc-pytorch/tree/main?tab=readme-ov-file#wav2letter-ctc-pytorch

cc: @cccclai @cbilgin

Versions

latest main

cc @cccclai @winskuo-quic @shewu-quic

Start with examples/qualcomm/scripts/wav2letter.py, especially get_dataset and the main entry point, then inspect the torchaudio.load failure for the LibriSpeech .flac path. Re-run the provided wav2letter command while checking the dataset and audio-loading setup; done means the script exports successfully with the stated model and options.