Add CudaQuantizer to onnxruntime.quantization (#29509)
## Description
Adds a CUDA-focused `CudaQuantizer` helper to ONNX Runtime's Python
quantization tools and exports it from `onnxruntime.quantization`. The
helper centralizes the CUDA quantization paths needed by ONNX Runtime
tests and `onnxruntime-genai` model building, covering both per-channel
QMoE weight packing and MatMulNBits blockwise formats without
duplicating that logic in test code.
This also updates the CUDA MoE/QMoE transformer tests to consume the
shared helper, including full-range symmetric offset storage and CUTLASS
mixed-GEMM prepacked layouts.
## Summary of Changes
### Quantization API
| File | Change |
|------|--------|
| `onnxruntime/python/tools/quantization/cuda_quantizer.py` | Adds
`CudaQuantizer` with per-channel INT4/INT8 quantization, CUDA mixed-GEMM
prepacking, MatMulNBits blockwise quantization, CUTLASS prepacked
blockwise quantization, and a pure-PyTorch symmetric blockwise reference
path. |
| `onnxruntime/python/tools/quantization/__init__.py` | Exports
`CudaQuantizer` from `onnxruntime.quantization`. |
| `onnxruntime/python/tools/quantization/qmoe_quantizer.py` | Replaced
by the more general CUDA quantizer implementation. |
### Test Updates
| File | Change |
|------|--------|
| `onnxruntime/test/python/transformers/test_moe_cuda.py` | Replaces
duplicated blockwise quantization/prepacking logic with `CudaQuantizer`
calls. |
| `onnxruntime/test/python/transformers/test_qmoe_cuda.py` | Uses
`CudaQuantizer` for per-channel and blockwise QMoE test setup, including
full-range unsigned offset storage coverage and default prepacked layout
smoke tests. |
## Testing
- Built the CUDA Release configuration.
- `pytest transformers/test_qmoe_cuda.py::TestQMoEIntPrePackSmoke` - 5
passed, 2 subtests passed.
- `pytest transformers/test_qmoe_cuda.py` - 97 passed, 12 skipped, 2
subtests passed.
- Validated without CUDA that importing the quantization module does not
require `torch`, and that torch-backed methods raise a clear error only
when used.
- `python -m py_compile
onnxruntime/python/tools/quantization/cuda_quantizer.py`
## Checklist
- [x] Tests added/updated
- [x] No breaking changes to released APIs
- [ ] CI passes
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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
