Add support for custom classes (#71)

* Support for custom classes

* Fix test

* Fix pre-commit

* Add pipeline logic for custom classes

* Fix minor issues, fix test

* Fix issues from PR review

* Fix tests

* Remove palette as str

* Rename old_to_new_ids to label_map

* Test for load_anns

* Remove get_palette function

* fixed temp

* Add subset of palette, remove palette as arg

* minor update

Co-authored-by: Jiarui XU <xvjiarui0826@gmail.com>