Add MLFLOW_FLATTEN_PARAMS support in MLflowCallback (#17148)

* add support for MLFLOW_FLATTEN_PARAMS

* ensure key is str

* fix style and update warning msg

* Empty commit to trigger CI

* fix bug in check_inits.py

* add unittest for flatten_dict utils

* fix 'NoneType' object is not callable on __del__

* add generic flatten_dict unittest to SPECIAL_MODULE_TO_TEST_MAP

* fix style

utils/check_inits.py

@@ -249,7 +249,7 @@ def get_transformers_submodules():
             if fname == "__init__.py":
                 continue
             short_path = str((Path(path) / fname).relative_to(PATH_TO_TRANSFORMERS))
-            submodule = short_path.replace(os.path.sep, ".").replace(".py", "")
+            submodule = short_path.replace(".py", "").replace(os.path.sep, ".")
             if len(submodule.split(".")) == 1:
                 submodules.append(submodule)
     return submodules