Disable O2 compile optimization for igamma ops

Author: toyxu

src/ATen/native/xpu/sycl/IGammaKernel.cpp

@@ -11,7 +11,7 @@ template <typename scalar_t>
 struct IgammaFunctor {
   IgammaFunctor(bool calc_igammac) : calc_igammac_(calc_igammac) {}
   bool calc_igammac_;
-  scalar_t operator()(scalar_t a, scalar_t b) const {
+  [[clang::optnone]] scalar_t operator()(scalar_t a, scalar_t b) const {
     if (calc_igammac_) {
       return calc_igammac<scalar_t>(a, b);
     } else {