[Bugfix][Hardware][AMD] Consolidate FP8 min/max values helper function (#31106)

Signed-off-by: c0de128 <kevin.mckay@outlook.com>
Signed-off-by: Kevin McKay <kevin@example.com>
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

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

@@ -19,6 +19,17 @@ FP8_DTYPE = current_platform.fp8_dtype()
 FP4_DTYPE = torch.uint8
 
 
+def get_fp8_min_max() -> tuple[float, float]:
+    """Get the min and max values for FP8 quantization."""
+    # Using the default value (240.0) from pytorch will cause accuracy
+    # issue on dynamic quantization models on ROCm. Here, use 224.0 for fnuz
+    # on ROCm platforms that use the torch.float8_e4m3fnuz dtype.
+    if current_platform.is_fp8_fnuz():
+        return -224.0, 224.0
+    finfo = torch.finfo(current_platform.fp8_dtype())
+    return finfo.min, finfo.max
+
+
 # Use proxy as NamedTuple direct subclasses cannot have static members
 class _GroupShape(NamedTuple):
     row: int