add dm.py into global libs

docs/source/_libs/dm.py

+import importlib.util
+import os
+from types import ModuleType
+
+
+def get_module(filename) -> ModuleType:
+    _, _simple_filename = os.path.split(filename)
+    _module_name, _ = _simple_filename.split('.', maxsplit=1)
+
+    spec = importlib.util.spec_from_file_location(_module_name, filename)
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+
+    return module

docs/source/tutorials/quick_start/complex_demo.py

-import importlib.util
 import os
-from functools import lru_cache
 
-_FILE_NAME = 'create_a_complex_tree.demo.py'
-_, _SIMPLE_FILE_NAME = os.path.split(_FILE_NAME)
-_MODULE_NAME, _ = _SIMPLE_FILE_NAME.split('.', maxsplit=1)
+from dm import get_module
 
-
-@lru_cache()
-def _get_module():
-    spec = importlib.util.spec_from_file_location(_MODULE_NAME, _FILE_NAME)
-    module = importlib.util.module_from_spec(spec)
-    spec.loader.exec_module(module)
-
-    return module
-
-
-_module = _get_module()
+_module = get_module(os.path.abspath('create_a_complex_tree.demo.py'))
 t1, t2, t3 = _module.t1, _module.t2, _module.t3

docs/source/tutorials/quick_start/simple_demo.py

-import importlib.util
 import os
-from functools import lru_cache
 
-_FILE_NAME = 'create_a_tree.demo.py'
-_, _SIMPLE_FILE_NAME = os.path.split(_FILE_NAME)
-_MODULE_NAME, _ = _SIMPLE_FILE_NAME.split('.', maxsplit=1)
+from dm import get_module
 
-
-@lru_cache()
-def _get_module():
-    spec = importlib.util.spec_from_file_location(_MODULE_NAME, _FILE_NAME)
-    module = importlib.util.module_from_spec(spec)
-    spec.loader.exec_module(module)
-
-    return module
-
-
-_module = _get_module()
+_module = get_module(os.path.abspath('create_a_tree.demo.py'))
 t = _module.t