[zero] reorganize zero/gemini folder structure (#3424)

* [zero] refactor low-level zero folder structure

* [zero] fix legacy zero import path

* [zero] fix legacy zero import path

* [zero] remove useless import

* [zero] refactor gemini folder structure

* [zero] refactor gemini folder structure

* [zero] refactor legacy zero import path

* [zero] refactor gemini folder structure

* [zero] refactor gemini folder structure

* [zero] refactor gemini folder structure

* [zero] refactor legacy zero import path

* [zero] fix test import path

* [zero] fix test

* [zero] fix circular import

* [zero] update import

colossalai/zero/low_level/bookkeeping/base_store.py

@@ -0,0 +1,17 @@
+import torch.distributed as dist
+from torch.distributed import ProcessGroup
+
+
+class BaseStore:
+
+    def __init__(self, torch_pg: ProcessGroup):
+        self._world_size = dist.get_world_size(group=torch_pg)
+        self._local_rank = dist.get_rank(group=torch_pg)
+
+    @property
+    def world_size(self):
+        return self._world_size
+
+    @property
+    def local_rank(self):
+        return self._local_rank