Add half->int8 saturate conversion to promise valid range (#1983)

* Add half->int8 saturate conversion to promise valid range

* add gpu only macro

---------

Co-authored-by: Haicheng Wu <haichengw@nvidia.com>

include/cutlass/numeric_conversion.h

@@ -267,6 +267,44 @@ struct NumericConverter<uint8_t, float, FloatRoundStyle::round_toward_zero> {
   }
 };
 
+/////////////////////////////////////////////////////////////////////////////////////////////////
+//
+// Partial specializations for cutlass::half_t => int8_t
+//
+/////////////////////////////////////////////////////////////////////////////////////////////////
+
+template <>
+struct NumericConverter<int8_t, cutlass::half_t, FloatRoundStyle::round_to_nearest> {
+
+  using result_type = int8_t;
+  using source_type = cutlass::half_t;
+  static FloatRoundStyle const round_style = FloatRoundStyle::round_to_nearest;
+
+  CUTLASS_HOST_DEVICE
+  static result_type convert(source_type const & s) {
+    #if defined(__CUDA_ARCH__)
+    union { int8_t int8[2]; int16_t int16; };
+    union { cutlass::half_t fp16; int16_t int16_in; };
+    fp16 = s;
+    asm volatile ("cvt.rni.sat.s8.f16 %0, %1;" : "=h"(int16) : "h"(int16_in));
+    return int8[0];
+    #elif !defined(__CUDACC_RTC__)
+    std::fesetround(FE_TONEAREST);
+    int32_t intermediate = (int32_t)std::nearbyint(static_cast<float>(s));
+    // Low-end saturation
+    intermediate = std::max(intermediate, (int32_t)std::numeric_limits<int8_t>::lowest());
+    // High-end saturation
+    intermediate = std::min(intermediate, (int32_t)std::numeric_limits<int8_t>::max());
+    return static_cast<result_type>(intermediate);
+    #endif
+  }
+
+  CUTLASS_HOST_DEVICE
+  result_type operator()(source_type const &s) const {
+    return convert(s);
+  }
+};
+
 /////////////////////////////////////////////////////////////////////////////////////////////////
 //
 // Partial specializations for float => integer_subbyte