[SYCL] Fix AccessorSubscript windows build issue (#6648)

This PR fixes an issue caused when building on Windows with MSVC. The problem was introduced in #6341. 
MSVC does not correctly infer the default template for its return type. This is fixed by setting the return type to auto and letting the compiler infer the return type.

Author: AidanBeltonS

sycl/include/sycl/accessor.hpp

@@ -1760,8 +1760,7 @@ class __SYCL_SPECIAL_CLASS accessor :
         multi_ptr<DataT, AS>(getQualifiedPtr() + LinearIndex));
   }
   template <int Dims = Dimensions, typename = detail::enable_if_t<(Dims > 1)>>
-  typename AccessorCommonT::template AccessorSubscript<Dims - 1>
-  operator[](size_t Index) const {
+  auto operator[](size_t Index) const {
     return AccessorSubscript<Dims - 1>(*this, Index);
   }