[coll] Define interface for bridging. (#9695)

* Define the basic interface that will shared by nccl, federated and native.

tests/cpp/collective/test_allreduce.cc

@@ -4,6 +4,7 @@
 #include <gtest/gtest.h>
 
 #include "../../../src/collective/allreduce.h"
+#include "../../../src/collective/coll.h"  // for Coll
 #include "../../../src/collective/tracker.h"
 #include "test_worker.h"  // for WorkerForTest, TestDistributed
 
@@ -47,6 +48,19 @@ class AllreduceWorker : public WorkerForTest {
       ASSERT_EQ(v, 1.5 * static_cast<double>(comm_.World())) << i;
     }
   }
+
+  void BitOr() {
+    Context ctx;
+    std::vector<std::uint32_t> data(comm_.World(), 0);
+    data[comm_.Rank()] = ~std::uint32_t{0};
+    auto pcoll = std::make_shared<Coll>();
+    auto rc = pcoll->Allreduce(&ctx, comm_, EraseType(common::Span{data.data(), data.size()}),
+                               ArrayInterfaceHandler::kU4, Op::kBitwiseOR);
+    ASSERT_TRUE(rc.OK()) << rc.Report();
+    for (auto v : data) {
+      ASSERT_EQ(v, ~std::uint32_t{0});
+    }
+  }
 };
 
 class AllreduceTest : public SocketTest {};
@@ -69,4 +83,13 @@ TEST_F(AllreduceTest, Sum) {
     worker.Acc();
   });
 }
+
+TEST_F(AllreduceTest, BitOr) {
+  std::int32_t n_workers = std::min(7u, std::thread::hardware_concurrency());
+  TestDistributed(n_workers, [=](std::string host, std::int32_t port, std::chrono::seconds timeout,
+                                 std::int32_t r) {
+    AllreduceWorker worker{host, port, timeout, n_workers, r};
+    worker.BitOr();
+  });
+}
 }  // namespace xgboost::collective