Don't split input data in federated mode (#8279)

Co-authored-by: Hyunsu Philip Cho <chohyu01@cs.washington.edu>

plugin/federated/federated_communicator.h

@@ -170,6 +170,12 @@ class FederatedCommunicator : public Communicator {
    */
   bool IsDistributed() const override { return true; }
 
+  /**
+   * \brief Get if the communicator is federated.
+   * \return True.
+   */
+  bool IsFederated() const override { return true; }
+
   /**
    * \brief Perform in-place allreduce.
    * \param send_receive_buffer Buffer for both sending and receiving data.