Add rudimentary barrier

mlx/distributed/sockets/sockets.cpp

@@ -385,6 +385,19 @@ Group Group::split(int color, int key) {
   throw std::runtime_error("Splitting not supported yet");
 }
 
+void Group::barrier() {
+  char buff[128];
+  std::memset(buff, 1, 128);
+
+  auto group = std::static_pointer_cast<GroupImpl>(raw_group());
+  int size = group->size();
+  int rank = group->rank();
+
+  for (int distance = 1; distance <= size / 2; distance *= 2) {
+    group->send_recv_sum<char>(buff, 128, rank ^ distance);
+  }
+}
+
 Group init(bool strict /* = false */) {
   static std::shared_ptr<GroupImpl> global_group = nullptr;