fallback for cuda < 12.8 (#2697)

mlx/backend/cuda/CMakeLists.txt

@@ -59,6 +59,11 @@ target_sources(
 add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/binary)
 add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/unary)
 
+# fp4 is not available on < 12.8
+if(CMAKE_CUDA_COMPILER_VERSION VERSION_LESS 12.8.0)
+  target_include_directories(mlx PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/quantized/)
+endif()
+
 if(CMAKE_CUDA_COMPILER_VERSION VERSION_GREATER_EQUAL 12.9.0)
   target_sources(
     mlx PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/gemms/cublas_gemm_batched_12_9.cu)

mlx/backend/cuda/quantized/cuda_fp4.h

@@ -0,0 +1,56 @@
+#pragma once
+
+struct __nv_fp4_e2m1 {
+  __device__ __nv_fp4_e2m1(float x) {
+    if (std::isnan(x)) {
+        __x = 0x7;
+        return;
+    }
+
+    const uint8_t sign_bit = (std::signbit(x)) ? 0x8 : 0x0;
+    x = std::abs(x);
+
+    if (x > 5.0f) {
+        __x = 0x7;
+    } else if (x >= 3.5f) {
+        __x = 0x6;
+    } else if (x > 2.5f) {
+        __x = 0x5;
+    } else if (x >= 1.75f) {
+        __x = 0x4;
+    } else if (x > 1.25f) {
+        __x = 0x3;
+    } else if (x >= 0.75f) {
+        __x = 0x2;
+    } else if (x > 0.25f) {
+        __x = 0x1;
+    } else {
+        __x = 0x0;
+    }
+    __x |= sign_bit;
+  }
+
+  __device__ operator float() {
+    static const float LUT[16] = {
+        0.0f,
+        0.5f,
+        1.0f,
+        1.5f,
+        2.0f,
+        3.0f,
+        4.0f,
+        6.0f,
+        -0.0f,
+        -0.5f,
+        -1.0f,
+        -1.5f,
+        -2.0f,
+        -3.0f,
+        -4.0f,
+        -6.0f
+    };
+
+    return LUT[__x];
+  }
+  uint8_t __x{0};
+};