Optimizations for MXFP8/NVFP4 dequantize kernels

[YigongQin]

Description

• Handle empty tensors in dequantize for CUDA graph compatibility
• Add swizzled scale support to the NVFP4 dequantize kernel, reusing the existing MXFP8 swizzle index computation
• Add C++ unit tests for both NVFP4 and MXFP8 dequantization (including swizzled scale variants)
• Fix to_cpu() and set_scale() in test infrastructure to correctly sync amax/scale for NVTE_NVFP4_1D_SCALING mode

Type of change

• Documentation change (change only to the documentation, either a fix or a new content)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Infra/Build change
• Code refactoring

Changes

Please list the changes introduced in this PR:

• Handle empty tensors in dequantize for CUDA graph compatibility — Early return when input has zero elements, avoiding kernel launches on empty tensors.
• Add GEMM-swizzled scale support to NVFP4 dequantize kernel — Template the kernel with WITH_GEMM_SWIZZLED_SCALES to support reading scales from swizzled layout, reusing the MXFP8 swizzle index computation.
• Add GEMM-swizzled scale support to MXFP8 dequantize kernel — Extend the MXFP8 dequantize kernel to handle swizzled scale inputs.
• Add C++ unit tests for NVFP4 dequantization — 21 tests for compact scales + 21 tests for swizzled scales, covering multiple sizes and output dtypes (fp32, bf16, fp16).
• Add C++ unit tests for MXFP8 dequantization with swizzled scales — New swizzled test suite for MXFP8.
• Fix to_cpu() to sync amax/scale for NVFP4 tensors — Previously only synced for NVTE_DELAYED_TENSOR_SCALING, causing the CPU reference to use stale amax=0.
• Fix set_scale() to work for NVFP4 tensors — Same condition fix, enabling the scale to be properly uploaded to GPU before quantization.
• Fix swizzled test ordering — Move from_cpu() before the FP4 data copy to prevent from_cpu() from overwriting the copied data with zeros.

Checklist:

• I have read and followed the contributing guidelines
• The functionality is complete
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes