Qualcomm AI Engine Direct - Adding QNN backend support for reflection_pad3d core ATen op

[qti-horodnic]

Summary

Added support for aten.reflection_pad3d on the QNN HTP backend via a decomposition pass into the index_select + cat operations.

Note that QNN HTP's native Pad op with MIRROR_REFLECT scheme only supports tensors up to rank 4. Since reflection_pad3d operates on 5D tensors [N, C, D, H, W], it cannot be delegated directly. Using 2d or 1d variants of this op in a decomposition is complicated, so this PR decomposes reflection_pad3d into index_select and cat, which are both fully supported at 5D rank on HTP.

This is done by:

1. index_select to gather mirrored indices along the dimension.
2. cat to concatenate the reflected slices with the original tensor.

Test plan

pytest backends/qualcomm/tests/rework/htp/op/v68/test.py -k "test_reflection_pad_3d" -v


python backends/qualcomm/tests/test_qnn_delegate.py TestQNNQuantizedOperator.test_qnn_backend_reflection_pad3d --soc_model SM8750 --host aisw-vm15-labsd --device 545ee4aa --build_folder build-android

python backends/qualcomm/tests/test_qnn_delegate.py TestQNNFloatingPointOperator.test_qnn_backend_reflection_pad3d --soc_model SM8750 --host aisw-vm15-labsd --device 545ee4aa --build_folder build-android

[pytorch-bot]

🔗 Helpful Links

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⚠️ 12 Awaiting Approval

As of commit 10f8be4 with merge base d632341 ():

AWAITING APPROVAL - The following workflows need approval before CI can run:

• Build documentation (gh)
• Build Presets (gh)
• Lint (gh)
• periodic (gh)
• pull (gh)
• Test ARM Backend (gh)
• Test CoreML Backend (gh)
• Test QNN Backend (gh)
• Test QNN Windows MSVC build (gh)
• Test Vulkan Backend (gh)
• Test WebGPU Backend (gh)
• Test XNNPACK Backend (gh)

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[qti-horodnic]

@pytorchbot label "release notes: qualcomm"