fix saturate

mlx/backend/cpu/unary_ops.h

@@ -140,7 +140,7 @@ struct ToFP8 {
     auto result_high = Simd<uint8_t, N>(f_bits_high >> 20);
     result = select(f_bits < (121 << 23), result_low, result_high);
 
-    auto result_sat = Simd<uint8_t, N>(fp8_max);
+    auto result_sat = Simd<uint8_t, N>(0x7E);
     result = select(f_bits >= fp8_max, result_sat, result);
     return result | Simd<uint8_t, N>(sign >> 24);
   }

mlx/backend/metal/kernels/unary_ops.h

@@ -459,7 +459,7 @@ struct ToFP8 {
     f_bits ^= sign;
     if (f_bits >= fp8_max) {
       // Default behavior saturates to min/max
-      result = fp8_max;
+      result = 0x7E;
     } else {
       if (f_bits < (121 << 23)) {
         f_bits =

mlx/backend/no_cpu/primitives.cpp

@@ -130,6 +130,7 @@ NO_CPU(View)
 
 namespace fast {
 NO_CPU_MULTI(Quantize)
+NO_CPU_MULTI(ConvertFP8)
 } // namespace fast
 
 namespace distributed {

mlx/backend/no_gpu/primitives.cpp

@@ -154,6 +154,7 @@ NO_GPU_USE_FALLBACK(RMSNorm)
 NO_GPU_MULTI(RMSNormVJP)
 NO_GPU_USE_FALLBACK(RoPE)
 NO_GPU(ScaledDotProductAttention)
+NO_GPU_MULTI(ConvertFP8)
 NO_GPU_MULTI(Quantize)
 NO_GPU_MULTI(CustomKernel)
 } // namespace fast