use numel() when appropriate in get_reduced_dim_product (#9142)

No need to re-calculate numel() here.

Author: swolchok

kernels/portable/cpu/util/reduce_util.cpp

@@ -83,12 +83,8 @@ size_t get_reduced_dim_product(
   if (in.dim() == 0) {
     return 1;
   }
-  size_t dim_product = 1;
   if (!dim.has_value()) {
-    for (size_t i = 0; i < static_cast<size_t>(in.dim()); ++i) {
-      dim_product *= in.size(i);
-    }
-    return dim_product;
+    return in.numel();
   }
   const size_t d = _normalize_non_neg_d(dim.value(), in.dim());
   return in.size(d);
@@ -104,16 +100,12 @@ size_t get_reduced_dim_product(
   if (in.dim() == 0) {
     return 1;
   }
-  size_t dim_product = 1;
-  const size_t in_dim = in.dim();
   if (!dim_list.has_value() || dim_list.value().size() == 0) {
-    for (size_t i = 0; i < static_cast<size_t>(in.dim()); ++i) {
-      dim_product *= in.size(i);
-    }
-    return dim_product;
+    return in.numel();
   }
+  size_t dim_product = 1;
   for (const auto& d : dim_list.value()) {
-    const size_t non_neg_d = _normalize_non_neg_d(d, in_dim);
+    const size_t non_neg_d = _normalize_non_neg_d(d, in.dim());
     dim_product *= in.size(non_neg_d);
   }
   return dim_product;