Multiclass detector implementation (#68)

* Update TODO

* TODO

* Support Halpe26

* Delete README.md

* Revert original README

* Update Readme and tiny other edits

* Update rtmlib/tools/solution/body_and_feet.py

Co-authored-by: Tau <taujiang@outlook.com>

* Rename body+feet_demo.py to body_with_feet_demo.py

* Rename body_and_feet.py to body_with_feet.py

* Rename

* Fix typo

* Update README.md

Co-authored-by: Tau <taujiang@outlook.com>

* Update README.md

Co-authored-by: Tau <taujiang@outlook.com>

* Tracking was not taken into account

I don't think tracking was taken into account. 
Reordered keypoints, scores, according to track_id

* - Added the Custom class, a more general solution which works with anydetection and/or pose estimation class
- Made PoseTracker work with the Custom class and even when no detection mode is used

* NMS for RTMO did not work

* replaced rtmw in balanced mode as the toes were wrongly detected at the head

* custom version until pull requests are accepted

* Updated docstring

* Revert to previous setup.py

* nms_thr and score_thr as optional parameters when calling rtmo

* ViTPose implementation

* solve merge conflicts

* solve last merge conflicts

* implement multiclass detector

* added models to readme

* Fixed multiclass tracking, completed Readme, fixed pose_tracker bugs

* last fix on tracker

* Should work but I need to further test tomorrow

* last edits

* Update rtmlib/tools/pose_estimation/vitpose.py

* Update rtmlib/visualization/skeleton/coco25.py

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Co-authored-by: HunMinKim <144449115+hunminkim98@users.noreply.github.com>
Co-authored-by: Kim HunMin <gns5758@inha.edu>
Co-authored-by: Tau <taujiang@outlook.com>