add categories metadata, add dataset name to each instance
add categories metadata, add dataset name to each instance

Summary:
1. Adds category metadata to keep the original mapping `category_id` -> `category_name`; this is required to correctly merge datasets with objects from different but compatible category sets;
2. Adds `dataset_name` entry for every annotation instance; this would allow one to combine annotation instances from datasets with different metadata and use the correct metadata in transforms, evaluations etc.

Reviewed By: patricklabatut

Differential Revision: D21227014

fbshipit-source-id: ff77ab9d11a262e10221d585bd86e7b9c1c1eeaa

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2