Eliminate the boring warning if no AOT enabled (#2180)

# Motivation
I would like to clarify that, according to the [FP64 Partial Emulation
Proposal](https://intel.sharepoint.com/:w:/s/MLTSHdGPU/EaroFZY371hOqNL9182g2_EBOe83qGYTriAavPB6WTWXYg?e=XSRnKt),
the SYCL compiler and IGC only perform FP64 conversion on the DG2 and
ATS-M architectures, and this is only available when AOT is enabled.

If AOT is not enabled, many warnings like the following will be emitted:
```bash
icx: warning: '-fsycl-fp64-conv-emu' option is supported only for AOT compilation of Intel GPUs. It will be ignored for other targets
```

To avoid these warnings, the `-fsycl-fp64-conv-emu` flag should only be
added when AOT is enabled for the specific target architectures.

Author: guangyey

cmake/BuildFlags.cmake

@@ -101,9 +101,7 @@ if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR CMAKE_CXX_COMPILER_ID STREQUAL "MSVC"
   endif()
 
   CHECK_SYCL_FLAG("-fsycl-fp64-conv-emu" SUPPORTS_FP64_CONV_EMU)
-  if(SUPPORTS_FP64_CONV_EMU)
-    set(SYCL_KERNEL_OPTIONS ${SYCL_KERNEL_OPTIONS} -fsycl-fp64-conv-emu)
-  else()
+  if(NOT SUPPORTS_FP64_CONV_EMU)
     message(WARNING "The compiler does not support the '-fsycl-fp64-conv-emu' flag, \
     will disable it. On some platforms that don't support FP64, \
     running operations with the FP64 datatype will raise a Runtime error: Required aspect fp64 is not supported on the device \
@@ -128,7 +126,6 @@ if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR CMAKE_CXX_COMPILER_ID STREQUAL "MSVC"
   set(SYCL_OFFLINE_COMPILER_CG_OPTIONS "${SYCL_OFFLINE_COMPILER_CG_OPTIONS} -options -cl-fp32-correctly-rounded-divide-sqrt")
   set(SYCL_OFFLINE_COMPILER_CG_OPTIONS "${SYCL_OFFLINE_COMPILER_CG_OPTIONS} -options -cl-intel-greater-than-4GB-buffer-required")
 
-
   if(WIN32)
     set(AOT_TARGETS "mtl,mtl-h,bmg,dg2,arl-h,lnl-m")
   else()
@@ -140,6 +137,14 @@ if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR CMAKE_CXX_COMPILER_ID STREQUAL "MSVC"
   if(AOT_TARGETS STREQUAL "none")
     set(TORCH_XPU_ARCH_LIST "" PARENT_SCOPE)
   else()
+    # Enable FP64 conversion emulation for DG2 / ATS-M targets
+    if(SUPPORTS_FP64_CONV_EMU)
+      string(FIND "${AOT_TARGETS}" "dg2" _dg2_index)
+      string(FIND "${AOT_TARGETS}" "ats-m" _atsm_index)
+      if(_dg2_index GREATER_EQUAL 0 OR _atsm_index GREATER_EQUAL 0)
+        set(SYCL_KERNEL_OPTIONS ${SYCL_KERNEL_OPTIONS} -fsycl-fp64-conv-emu)
+      endif()
+    endif()
     set(SYCL_TARGETS_OPTION -fsycl-targets=spir64_gen,spir64)
     set(SYCL_KERNEL_OPTIONS ${SYCL_KERNEL_OPTIONS} ${SYCL_TARGETS_OPTION})
     set(SYCL_DEVICE_LINK_FLAGS ${SYCL_DEVICE_LINK_FLAGS} ${SYCL_TARGETS_OPTION})