[Fix] Improve MultiImageMixDataset unittests coverage (#1197)

* Fix typo in usage example

* original MultiImageMixDataset code in mmdet

* Add MultiImageMixDataset unittests in test_dataset_wrapper

* fix lint error

* fix value name ann_file to ann_dir

* modify retrieve_data_cfg (#1)

* remove dynamic_scale & add palette

* modify retrieve_data_cfg method

* modify retrieve_data_cfg func

* fix error

* improve the unittests coverage

* fix unittests error

* Dataset (#2)

* add cfg-options

* Add unittest in test_build_dataset

* add blank line

* add blank line

* add a blank line

Co-authored-by: Miao Zheng <76149310+MeowZheng@users.noreply.github.com>

* [Fix] Add MultiImageMixDataset unittests

Co-authored-by: Younghoon-Lee <72462227+Younghoon-Lee@users.noreply.github.com>
Co-authored-by: MeowZheng <meowzheng@outlook.com>
Co-authored-by: Miao Zheng <76149310+MeowZheng@users.noreply.github.com>