[Feature] Add Mosaic transform (#1093)

* Fix typo in usage example

* original mosaic code in mmdet

* Adjust mosaic to the semantic segmentation

* Remove bbox test in test_mosaic

* Add unittests

* Fix resize mode for seg_fields

* Fix repr error

* modify Mosaic docs

* modify from Mosaic to RandomMosaic

* Add docstring

* modify Mosaic docstring

* [Docs] Add a blank line before Returns:

* add blank lines

Co-authored-by: MeowZheng <meowzheng@outlook.com>