* [Enhance] Ensure metrics is not empty when saving best ckpts

* fix warn to warning

* delete a unnecessary method

* [Refactor] Refactor fileio but without breaking bc

* handle compatibility

* fix format

* modify io functions

* fix ut

* fix ut

* rename method names

* refine

* refine docstring

* fix ut in windows

* update ut

* minor fix

* ensure client is not None when closing it

* add more examples for list_dir_or_file interface

* refine docstring

* refine deprecated info

* fix ut

* add a description for lmdb docstring

* fix wrong epoch andd iter when save bbest ckpt

* fix ut

* fix resume best ckpt unexpectedly

* minor refine

* fix unit test

* support save best based on multi metrics

* add unit test

* resolve bugs after rebasing

* revise docstring

* revise docstring

* fix as comment

* revise as comment

* fix bug of not save-best in iteration-based training

* revise the unit test

* [Feat] Support save best ckpt

* reformat code

* rename function and reformat code

* fix logging info

* support resume from ceph

* move func and refine

* delete symlink

* fix unittest

* perserve _allow_symlink and symlink

* change CheckPointHook before_run to before train

* using tmp_path in each checkpointhook test case

* [Refactor] Add batch_idx to hook input.

* update

* [Feature]: Add dist semantics in checkpoint hook

* [Fix]: Delete sync buffer in checkpoint hook

* [Feature]: Add checkpoint hook

* [Fix]: Fix lint

* [Fix]: Delete redundant optional and give an example to our_dir

* [Feature]: Add test the last_ckpt in UT

* [Fix]: Fix docstring problem

* [Fix]: Add patch to UT

* [Feature]: Add Test case for by epoch