use positional arg for mapper=
use positional arg for mapper=

Summary:
this matches old cfg-based behavior

close https://github.com/facebookresearch/detectron2/issues/2291

Reviewed By: alexander-kirillov

Differential Revision: D25097215

fbshipit-source-id: 6589c3a5560bcbe5e655db13450e0fa1818512c0

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2