Make reference code zero points float instead of int.

These get converted to float anyways, and this is more flexible.

PiperOrigin-RevId: 705259824

src/xnnpack/microparams.h

@@ -589,9 +589,9 @@ struct xnn_x32_packb_params {
 
 struct xnn_unary_reference_params {
   float x_scale;
-  int32_t x_zero_point;
+  float x_zero_point;
   float inv_y_scale;
-  int32_t y_zero_point;
+  float y_zero_point;
   union xnn_unary_params params;
 };
 

src/xnnpack/reference-utils.h

@@ -42,12 +42,12 @@ Result round_float_to_int(float x) {
 }
 
 template <typename T>
-float dequantize(T x, float scale, int32_t zero_point) {
-  return (static_cast<float>(x) - static_cast<float>(zero_point)) * scale;
+float dequantize(T x, float scale, float zero_point) {
+  return (static_cast<float>(x) - zero_point) * scale;
 }
 
 template <typename T>
-T quantize(float x, float inv_scale, int32_t zero_point) {
+T quantize(float x, float inv_scale, float zero_point) {
   return round_float_to_int<T>(x * inv_scale + zero_point);
 }