Fix inverse in fake quant (#36762)

paddle/fluid/operators/fake_quantize_op.cu

@@ -216,14 +216,14 @@ __global__ void ClipAndQuantDequantKernel(const T* in, const T* scale,
   int tid = threadIdx.x;
 
   T s = scale[0];
+  T inv_s = inverse(s);
   T bin_cnt_t = static_cast<T>(bin_cnt);
 
   for (int i = bid; i < n; i += blockDim.x * gridDim.x) {
     T x = in[i];
     x = x > s ? s : x;
     x = x < -s ? -s : x;
-    x = (bin_cnt_t / s) * x;
-
+    x = bin_cnt_t * inv_s * x;
     x = static_cast<T>(round(static_cast<float>(x)));
     out[i] = (x * s) / bin_cnt_t;
   }

paddle/fluid/operators/fake_quantize_op.h

@@ -28,8 +28,9 @@ namespace operators {
 
 template <typename T>
 inline HOSTDEVICE T inverse(T s) {
-  T eps = 1e-6;
-  return s <= 1e-30 ? 1.0 / (s + eps) : 1.0 / s;
+  T eps = static_cast<T>(1e-6);
+  T one = static_cast<T>(1.0);
+  return s <= static_cast<T>(1e-30) ? one / (s + eps) : one / s;
 }
 
 template <typename DeviceContext, typename T>