[Breaking] Switch from rabit to the collective communicator (#8257)

* Switch from rabit to the collective communicator

* fix size_t specialization

* really fix size_t

* try again

* add include

* more include

* fix lint errors

* remove rabit includes

* fix pylint error

* return dict from communicator context

* fix communicator shutdown

* fix dask test

* reset communicator mocklist

* fix distributed tests

* do not save device communicator

* fix jvm gpu tests

* add python test for federated communicator

* Update gputreeshap submodule

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

jvm-packages/xgboost4j/src/test/java/ml/dmlc/xgboost4j/java/DMatrixTest.java

@@ -300,7 +300,7 @@ public class DMatrixTest {
   public void testTrainWithDenseMatrixRef() throws XGBoostError {
     Map<String, String> rabitEnv = new HashMap<>();
     rabitEnv.put("DMLC_TASK_ID", "0");
-    Rabit.init(rabitEnv);
+    Communicator.init(rabitEnv);
     DMatrix trainMat = null;
     BigDenseMatrix data0 = null;
     try {
@@ -348,7 +348,7 @@ public class DMatrixTest {
       else if (data0 != null) {
         data0.dispose();
       }
-      Rabit.shutdown();
+      Communicator.shutdown();
     }
   }