move tridentnet models to "TridentNet/"
move tridentnet models to "TridentNet/"

Summary: other subdirectories are also capitalized

Reviewed By: rbgirshick, lyttonhao

Differential Revision: D18728209

fbshipit-source-id: 215505a70c4c77f7351aa9356b43296780c99b26

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2