[coll] allgather. (#9681)

src/collective/allgather.h

@@ -20,4 +20,24 @@ namespace cpu_impl {
                                    std::shared_ptr<Channel> prev_ch,
                                    std::shared_ptr<Channel> next_ch);
 }  // namespace cpu_impl
+
+template <typename T>
+[[nodiscard]] Result RingAllgather(Comm const& comm, common::Span<T> data, std::size_t size) {
+  auto n_total_bytes = data.size_bytes();
+  auto n_bytes = sizeof(T) * size;
+  auto erased =
+      common::Span<std::int8_t>{reinterpret_cast<std::int8_t*>(data.data()), n_total_bytes};
+
+  auto rank = comm.Rank();
+  auto prev = BootstrapPrev(rank, comm.World());
+  auto next = BootstrapNext(rank, comm.World());
+
+  auto prev_ch = comm.Chan(prev);
+  auto next_ch = comm.Chan(next);
+  auto rc = cpu_impl::RingAllgather(comm, erased, n_bytes, 0, prev_ch, next_ch);
+  if (!rc.OK()) {
+    return rc;
+  }
+  return comm.Block();
+}
 }  // namespace xgboost::collective