[polish] polish singleton and global context (#500)

colossalai/context/parallel_context.py

@@ -15,30 +15,16 @@ from colossalai.registry import DIST_GROUP_INITIALIZER
 
 from .parallel_mode import ParallelMode
 from .random import add_seed, get_seeds, set_mode
+from colossalai.context.singleton_meta import SingletonMeta
 
 
-class ParallelContext:
+class ParallelContext(metaclass=SingletonMeta):
     """This class provides interface functions for users to get the parallel context,
     such as the global rank, the local rank, the world size, etc. of each device.
 
     """
 
-    __instance = None
-
-    @staticmethod
-    def get_instance():
-        if ParallelContext.__instance is None:
-            ParallelContext()
-        return ParallelContext.__instance
-
     def __init__(self):
-        # create a singleton instance
-        if ParallelContext.__instance is not None:
-            raise Exception(
-                'ParallelContext is a singleton class, you should get the instance by colossalai.core.global_context')
-        else:
-            ParallelContext.__instance = self
-
         # distributed settings
         self._global_ranks = dict()
         self._local_ranks = dict()
@@ -510,3 +496,6 @@ class ParallelContext:
 
     def set_virtual_pipeline_parallel_rank(self, rank):
         self.virtual_pipeline_parallel_rank = rank
+
+
+global_context = ParallelContext()