[pipeline] add stage manager (#4093)

* [pipeline] add stage manager

* [test] add pipeline stage manager test

* [pipeline] add docstring for stage manager

tests/test_pipeline/test_stage_manager.py

@@ -21,7 +21,7 @@ def check_stage_manager():
         1: [0, 1],
         2: [2, 3],
         3: [2, 3],
-    }   
+    }
     pg_mesh = ProcessGroupMesh(DP_SIZE, PP_SIZE)
     stage_manager = PipelineStageManager(pg_mesh, PP_DIM)
     rank = dist.get_rank()