Boilerplate for custom HashMap implementation (#373)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

pyg_lib/csrc/classes/cpu/hash_map.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include <torch/library.h>
+
+namespace pyg {
+namespace classes {
+
+struct CPUHashMap : torch::CustomClassHolder {
+  CPUHashMap() {};
+};
+
+TORCH_LIBRARY(pyg, m) {
+  m.class_<CPUHashMap>("CPUHashMap").def(torch::init());
+}
+
+}  // namespace classes
+}  // namespace pyg

test/csrc/classes/test_hash_map.cpp

@@ -0,0 +1,7 @@
+#include <gtest/gtest.h>
+
+#include "pyg_lib/csrc/classes/cpu/hash_map.h"
+
+TEST(CPUHashMapTest, BasicAssertions) {
+  auto map = pyg::classes::CPUHashMap();
+}