[Optimization] Replaced std::unordered_map by phmap::flat_hash_map (#1662)

Co-authored-by: Da Zheng <zhengda1936@gmail.com>

.gitmodules

@@ -13,3 +13,6 @@
 [submodule "third_party/METIS"]
 	path = third_party/METIS
 	url = https://github.com/KarypisLab/METIS.git
+[submodule "third_party/phmap"]
+	path = third_party/phmap
+	url = https://github.com/greg7mdp/parallel-hashmap.git

src/array/cpu/array_utils.h

@@ -11,6 +11,7 @@
 #include <unordered_map>
 #include <utility>
 #include "../../c_api_common.h"
+#include "../third_party/phmap/parallel_hashmap/phmap.h"
 
 namespace dgl {
 namespace aten {
@@ -51,7 +52,7 @@ class IdHashMap {
     const int64_t len = ids->shape[0];
     for (int64_t i = 0; i < len; ++i) {
       const IdType id = ids_data[i];
-      // std::unorderd_map::insert assures that an insertion will not happen if the
+      // phmap::flat_hash_map::insert assures that an insertion will not happen if the
       // key already exists.
       oldv2newv_.insert({id, oldv2newv_.size()});
       filter_[id & kFilterMask] = true;
@@ -105,7 +106,7 @@ class IdHashMap {
   // Hashtable is very slow. Using bloom filter can significantly speed up lookups.
   std::vector<bool> filter_;
   // The hashmap from old vid to new vid
-  std::unordered_map<IdType, IdType> oldv2newv_;
+  phmap::flat_hash_map<IdType, IdType> oldv2newv_;
 };
 
 /*