* Support to use `indices` to specify which samples to evaluate.
* Add KFoldDataset wrapper
* Rename 'K' to 'num_splits' accroding to sklearn
* Add `kfold-cross-valid.py`
* Add unit tests
* Add help doc and docstring
* sampler bugfixes
* sampler bugfixes
* reorganize code
* minor fixes
* reorganize code
* minor fixes
* Use `mmcv.runner.get_dist_info` instead of `dist` package to get rank
and world size.
* Add `build_dataloader` unit tests and fix sampler's unit tests.
* Fix unit tests
* Fix unit tests
Co-authored-by: mzr1996 <mzr1996@163.com>