Adds checkpoint taskgroups to the online trainer.
Adds checkpoint taskgroups to the online trainer.

Summary:
1. Uses the upload_builder in the offline training.
2. Adds the checkpoint taskgroups to the online trainer.
3. Changes the naming rules so that the model checkpoint has the format of
<directory>/<entity_id>_<snapshot_id>.<node_name>.<snapshot_id>

Reviewed By: rayleichen

Differential Revision: D5665068

fbshipit-source-id: a8103aed2ca195a506174d2a1d50611d2f1d9c35

Yangqing/caffe2,davinwang/caffe2,davinwang/caffe2,pietern/caffe2,Yangqing/caffe2,sf-wind/caffe2,Yangqing/caffe2,sf-wind/caffe2,davinwang/caffe2,sf-wind/caffe2,pietern/caffe2,pietern/caffe2,pietern/caffe2,caffe2/caffe2,xzturn/caffe2,sf-wind/caffe2,xzturn/caffe2,xzturn/caffe2,sf-wind/caffe2,Yangqing/caffe2,xzturn/caffe2,pietern/caffe2,Yangqing/caffe2,davinwang/caffe2,davinwang/caffe2,xzturn/caffe2