fix loss_weight for self_supervision
Summary: previously loss_weight is not used correctly for self-supervision branch
Test Plan: buck test mode/dev-nosan //caffe2/caffe2/fb/dper/layer_models/models/experimental/tests:tum_test
Reviewed By: xianjiec
Differential Revision: D17862312
fbshipit-source-id: 554b793a5caa3886946c54333c81a0d8a10230d9
