Fix portable is[inf|nan|_out compilation on older Linux (#3272)

Summary:
By wrapping a potentially non-compliant `isinf`/`isnan` implementations into a lambda with a defined return type

Compiler should be able to optimize it out into direct function call, see https://godbolt.org/z/bqYGd47Mx

Pull Request resolved: #3272

Reviewed By: GregoryComer

Differential Revision: D56504717

Pulled By: malfet

fbshipit-source-id: 72da456027dbc837c3cfac83b18a5f002fedc3a5

Author: malfet

kernels/portable/cpu/op_isinf.cpp

@@ -15,8 +15,10 @@ namespace executor {
 namespace native {
 
 Tensor& isinf_out(RuntimeContext& ctx, const Tensor& in, Tensor& out) {
+  // Lambda is syntactic sugar needed to workaround compilation on some older
+  // non-compatible distros where isnan is returning int rather than bool
   return internal::unary_ufunc_realhb_to_bool(
-      static_cast<bool (*)(double)>(std::isinf), ctx, in, out);
+      [](double x) -> bool { return std::isinf(x); }, ctx, in, out);
 }
 
 } // namespace native

kernels/portable/cpu/op_isnan.cpp

@@ -15,8 +15,10 @@ namespace executor {
 namespace native {
 
 Tensor& isnan_out(RuntimeContext& ctx, const Tensor& in, Tensor& out) {
+  // Lambda is syntactic sugar needed to workaround compilation on some older
+  // non-compatible distros where isnan is returning int rather than bool
   return internal::unary_ufunc_realhb_to_bool(
-      static_cast<bool (*)(double)>(std::isnan), ctx, in, out);
+      [](double x) -> bool { return std::isnan(x); }, ctx, in, out);
 }
 
 } // namespace native