[fbsync] Add stereo matching losses (#6554)

Summary:
* Moved more losses into classes

* Added photometric loss

* quick fix for ssim loss return value

* added references

* replaced with unsqueeze

* renaming variables

* add ref to consistency loss

* made mask optional everywhere. generalised photometric displacement

* smoothness typo

* fixed flow channel selection bug

* aligned with training script

Reviewed By: NicolasHug

Differential Revision: D39765303

fbshipit-source-id: 59fb6a5e0248d126b15390f2af91c5fc6f121909

Co-authored-by: Joao Gomes <[email protected]>

Author: jdsgomes