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

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

src/collective/communicator-inl.h

@@ -88,6 +88,13 @@ inline int GetWorldSize() { return Communicator::Get()->GetWorldSize(); }
  */
 inline bool IsDistributed() { return Communicator::Get()->IsDistributed(); }
 
+/*!
+ * \brief Get if the communicator is federated.
+ *
+ * \return True if the communicator is federated.
+ */
+inline bool IsFederated() { return Communicator::Get()->IsFederated(); }
+
 /*!
  * \brief Print the message to the communicator.
  *

src/collective/communicator.h

@@ -78,6 +78,9 @@ class Communicator {
   /** @brief Whether the communicator is running in distributed mode. */
   virtual bool IsDistributed() const = 0;
 
+  /** @brief Whether the communicator is running in federated mode. */
+  virtual bool IsFederated() const = 0;
+
   /**
    * @brief Combines values from all processes and distributes the result back to all processes.
    *

src/collective/noop_communicator.h

@@ -16,6 +16,7 @@ class NoOpCommunicator : public Communicator {
  public:
   NoOpCommunicator() : Communicator(1, 0) {}
   bool IsDistributed() const override { return false; }
+  bool IsFederated() const override { return false; }
   void AllReduce(void *send_receive_buffer, std::size_t count, DataType data_type,
                  Operation op) override {}
   void Broadcast(void *send_receive_buffer, std::size_t size, int root) override {}

src/collective/rabit_communicator.h

@@ -53,6 +53,8 @@ class RabitCommunicator : public Communicator {
 
   bool IsDistributed() const override { return rabit::IsDistributed(); }
 
+  bool IsFederated() const override { return false; }
+
   void AllReduce(void *send_receive_buffer, std::size_t count, DataType data_type,
                  Operation op) override {
     switch (data_type) {