DefaultTrainer: compose SimpleTrainer instead of inheritance
DefaultTrainer: compose SimpleTrainer instead of inheritance

Summary: allow composing other trainers (next diff)

Reviewed By: theschnitz

Differential Revision: D24525208

fbshipit-source-id: ac9cc5dc22f550edb5c34815436c336770ad6058

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2