Reduce build warnings in kernel templates (#1839)

The following warnings are removed:
- Unused var in: src/ATen/native/xpu/sycl/GroupReduceUtils.h
- Comparison warning in src/ATen/native/xpu/sycl/pstl/PSTLFunctions.h

Author: xytintel

src/ATen/native/xpu/sycl/GroupReduceUtils.h

@@ -49,7 +49,6 @@ inline T& GroupReduceSumWithoutBroadcast(
     T& val,
     shared_t shared) {
   auto sg = item.get_sub_group();
-  int g_tid = item.get_local_linear_id();
   int sg_tid = sg.get_local_linear_id();
   int sg_id = sg.get_group_linear_id();
   int n_sg = get_local_linear_range<DIM>(item) / SIMD;

src/ATen/native/xpu/sycl/pstl/PSTLFunctions.h

@@ -357,7 +357,7 @@ struct InclusiveScanIfKernelFunctor {
     d_first_[item_id] = static_cast<T>(first_[item_id]);
 
     if (mask_ptr_[item_id] == 0) {
-      for (int64_t _k = 1; item_id - _k >= 0; _k++) {
+      for (int64_t _k = 1; (int64_t)item_id - _k >= 0; _k++) {
         auto tmp = first_[item_id - _k];
         d_first_[item_id] = static_cast<T>(p_(d_first_[item_id], tmp));
         if (mask_ptr_[item_id - _k] != 0)