[Kernel]Support W4A8 Grouped GEMM on Hopper (#29691)

Signed-off-by: czhu-cohere <conway.zhu@cohere.com>

vllm/model_executor/layers/quantization/utils/quant_utils.py

@@ -2,7 +2,7 @@
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 """This file is used for /tests and /benchmarks"""
 
-from collections.abc import Mapping
+from collections.abc import Callable, Mapping
 from dataclasses import dataclass
 from types import MappingProxyType
 from typing import ClassVar, NamedTuple
@@ -691,3 +691,51 @@ def cutlass_fp4_supported() -> bool:
     capability_tuple = current_platform.get_device_capability()
     capability = -1 if capability_tuple is None else capability_tuple.to_int()
     return cutlass_scaled_mm_supports_fp4(capability)
+
+
+def convert_bf16_scales_to_fp8(
+    quant_fp8: Callable, scales: torch.Tensor
+) -> tuple[torch.Tensor, torch.Tensor]:
+    """
+    Convert a BF16 scale tensor into the pair of (fp8_scales, channel_scales)
+    expected by W4A8 GEMM kernels.
+    """
+    assert scales.is_contiguous(), (
+        f"scale tensor must be contiguous, got {scales.stride()=}"
+    )
+    assert scales.is_cuda, "scales must be on gpu"
+
+    orig_shape = scales.shape
+    k_groups = orig_shape[-1]
+    flat_scales = scales.view(-1, k_groups)
+
+    fp8_scales, chan_scales = quant_fp8(flat_scales)
+    fp8_scales = (fp8_scales.float() / 8.0).to(torch.float8_e4m3fn)
+    chan_scales *= 8.0
+
+    # restore original shape
+    fp8_scales = fp8_scales.view(orig_shape)
+    chan_scales = chan_scales.view(orig_shape[:-1], -1)
+
+    return fp8_scales, chan_scales
+
+
+def convert_packed_uint4b8_to_signed_int4_inplace(t: torch.Tensor) -> torch.Tensor:
+    """
+    Convert int4b8 (packed to int32) to signed int4
+    """
+    assert t.is_cuda, "tensor must be on gpu"
+    assert t.dtype == torch.int32, f"expected int32 packed weights but got {t.dtype}"
+
+    # loop through the 8 4-bit nibbles in each int32 entry
+    for i in range(8):
+        shift = 4 * i
+        # extract the i-th 4-bit nibble
+        nib = (t >> shift) & 0xF
+        # clear the original nibble by masking out
+        t &= ~(0xF << shift)
+        # convert int4b8 [0..15] to signed int4 [-8..7] by subtracting 8
+        # and update in-place
+        t |= ((nib - 8) & 0xF) << shift
+
+    return t