Failed to export ViT model to QNN with quant_dtype set to None

🐛 Describe the bug

I can run this command successfully.

python -m examples.qualcomm.scripts.torchvision_vit -b build-android -m SM8650 --compile_only

However, if I set quant_dtype to None, I got following errors:

[ERROR] [Qnn ExecuTorch]: graph_prepare.cc:213:ERROR:could not create op: q::ConvLayer.fp16.s1.tcm

[ERROR] [Qnn ExecuTorch]: graph_prepare.cc:1403:ERROR:Op 0x2c76300000035 preparation failed with err:-1

[ERROR] [Qnn ExecuTorch]:  <E> "aten_view_copy_default_2" generated: could not create op

[ERROR] [Qnn ExecuTorch]:  <E> RouterX86 graph prepare failed 12

[ERROR] [Qnn ExecuTorch]:  <E> Failed to finalize graph (id: 1) with err 1002

[ERROR] [Qnn ExecuTorch]: Failed to finalize Qnn Graph with error: 1002
Traceback (most recent call last):
  File "/home/user/Projects/android/executorch/examples/qualcomm/scripts/torchvision_vit.py", line 150, in <module>
    main(args)
  File "/home/user/Projects/android/executorch/examples/qualcomm/scripts/torchvision_vit.py", line 75, in main
    build_executorch_binary(
  File "/home/user/Projects/android/executorch/examples/qualcomm/utils.py", line 294, in build_executorch_binary
    exported_program = to_backend(edge_prog.exported_program, qnn_partitioner)
  File "/home/user/.conda/envs/robot/lib/python3.10/functools.py", line 889, in wrapper
    return dispatch(args[0].__class__)(*args, **kw)
  File "/home/user/Projects/android/executorch/exir/backend/backend_api.py", line 396, in _
    tagged_graph_module = _partition_and_lower(
  File "/home/user/Projects/android/executorch/exir/backend/backend_api.py", line 319, in _partition_and_lower
    partitioned_module = _partition_and_lower_one_graph_module(
  File "/home/user/Projects/android/executorch/exir/backend/backend_api.py", line 249, in _partition_and_lower_one_graph_module
    lowered_submodule = to_backend(
  File "/home/user/.conda/envs/robot/lib/python3.10/functools.py", line 889, in wrapper
    return dispatch(args[0].__class__)(*args, **kw)
  File "/home/user/Projects/android/executorch/exir/backend/backend_api.py", line 113, in _
    preprocess_result: PreprocessResult = cls.preprocess(
  File "/home/user/Projects/android/executorch/backends/qualcomm/qnn_preprocess.py", line 111, in preprocess
    assert len(qnn_context_binary) != 0, "Failed to generate Qnn context binary."
AssertionError: Failed to generate Qnn context binary.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/user/.conda/envs/robot/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/home/user/.conda/envs/robot/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/home/user/Projects/android/executorch/examples/qualcomm/scripts/torchvision_vit.py", line 156, in <module>
    raise Exception(e)
Exception: Failed to generate Qnn context binary.
[INFO] [Qnn ExecuTorch]: Destroy Qnn context
[INFO] [Qnn ExecuTorch]: Destroy Qnn device
[INFO] [Qnn ExecuTorch]: Destroy Qnn backend

Versions

Python: 3.10.16
ExecuTorch: 0.4
QNN SDK: 2.29.0.241129

cc @cccclai @winskuo-quic @shewu-quic

Start with examples/qualcomm/scripts/torchvision_vit.py and reproduce the failure using the provided command with quant_dtype set to None. Trace the call through examples/qualcomm/utils.py into backends/qualcomm/qnn_preprocess.py, focusing on QNN context generation and the reported ConvLayer and view-copy errors. Done means the ViT export generates a QNN context binary successfully with quant_dtype=None.