[Refactor] Add -fp4-gemm-backend to replace SGLANG_FLASHINFER_FP4_GEMM_BACKEND (#16534)

Co-authored-by: Vincent Zhong <207368749+vincentzed@users.noreply.github.com>

python/sglang/srt/layers/quantization/fp4_utils.py

@@ -0,0 +1,70 @@
+from __future__ import annotations
+
+import logging
+from enum import Enum
+from typing import TYPE_CHECKING
+
+from sglang.srt.environ import envs
+
+if TYPE_CHECKING:
+    from sglang.srt.server_args import ServerArgs
+
+logger = logging.getLogger(__name__)
+
+
+class Fp4GemmRunnerBackend(Enum):
+    """Enum for FP4 GEMM runner backend selection."""
+
+    AUTO = "auto"
+    CUDNN = "cudnn"
+    CUTLASS = "cutlass"
+    TRTLLM = "trtllm"
+
+    def is_auto(self) -> bool:
+        return self == Fp4GemmRunnerBackend.AUTO
+
+    def is_cudnn(self) -> bool:
+        return self == Fp4GemmRunnerBackend.CUDNN
+
+    def is_cutlass(self) -> bool:
+        return self == Fp4GemmRunnerBackend.CUTLASS
+
+    def is_trtllm(self) -> bool:
+        return self == Fp4GemmRunnerBackend.TRTLLM
+
+
+FP4_GEMM_RUNNER_BACKEND: Fp4GemmRunnerBackend | None = None
+
+
+def initialize_fp4_gemm_config(server_args: ServerArgs) -> None:
+    """Initialize FP4 GEMM configuration from server args."""
+    global FP4_GEMM_RUNNER_BACKEND
+
+    backend = server_args.fp4_gemm_runner_backend
+
+    # Handle deprecated env var for backward compatibility
+    # TODO: Remove this in a future version
+    if envs.SGLANG_FLASHINFER_FP4_GEMM_BACKEND.is_set():
+        env_backend = envs.SGLANG_FLASHINFER_FP4_GEMM_BACKEND.get()
+        if backend == "auto":
+            logger.warning(
+                "SGLANG_FLASHINFER_FP4_GEMM_BACKEND is deprecated. "
+                f"Please use '--fp4-gemm-backend={env_backend}' instead."
+            )
+            backend = env_backend
+        else:
+            logger.warning(
+                f"FP4 GEMM backend set to '{backend}' via --fp4-gemm-backend overrides "
+                "environment variable SGLANG_FLASHINFER_FP4_GEMM_BACKEND. "
+                "Using server argument value."
+            )
+
+    FP4_GEMM_RUNNER_BACKEND = Fp4GemmRunnerBackend(backend)
+
+
+def get_fp4_gemm_runner_backend() -> Fp4GemmRunnerBackend:
+    """Get the current FP4 GEMM runner backend."""
+    global FP4_GEMM_RUNNER_BACKEND
+    if FP4_GEMM_RUNNER_BACKEND is None:
+        FP4_GEMM_RUNNER_BACKEND = Fp4GemmRunnerBackend.AUTO
+    return FP4_GEMM_RUNNER_BACKEND