[coll] Improvements and fixes for tracker and allreduce. (#9745)

- Allow the tracker to wait.
- Fix allreduce type cast
- Return args from the federated tracker.

tests/cpp/collective/test_tracker.cc

@@ -27,9 +27,15 @@ class PrintWorker : public WorkerForTest {
 
 TEST_F(TrackerTest, Bootstrap) {
   RabitTracker tracker{host, n_workers, 0, timeout};
+  ASSERT_FALSE(tracker.Ready());
   auto fut = tracker.Run();
 
   std::vector<std::thread> workers;
+
+  auto args = tracker.WorkerArgs();
+  ASSERT_TRUE(tracker.Ready());
+  ASSERT_EQ(get<String const>(args["DMLC_TRACKER_URI"]), host);
+
   std::int32_t port = tracker.Port();
 
   for (std::int32_t i = 0; i < n_workers; ++i) {
@@ -47,6 +53,9 @@ TEST_F(TrackerTest, Print) {
   auto fut = tracker.Run();
 
   std::vector<std::thread> workers;
+  auto rc = tracker.WaitUntilReady();
+  ASSERT_TRUE(rc.OK());
+
   std::int32_t port = tracker.Port();
 
   for (std::int32_t i = 0; i < n_workers; ++i) {