Move singleton to common directory (#10935)

### What problem does this PR solve?

As title

### Type of change

- [x] Refactoring

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

rag/utils/__init__.py

@@ -15,23 +15,10 @@
 #
 
 import os
-
 import tiktoken
 
 from api.utils.file_utils import get_project_base_directory
 
-
-def singleton(cls, *args, **kw):
-    instances = {}
-
-    def _singleton():
-        key = str(cls) + str(os.getpid())
-        if key not in instances:
-            instances[key] = cls(*args, **kw)
-        return instances[key]
-
-    return _singleton
-
 tiktoken_cache_dir = get_project_base_directory()
 os.environ["TIKTOKEN_CACHE_DIR"] = tiktoken_cache_dir
 # encoder = tiktoken.encoding_for_model("gpt-3.5-turbo")