Remove contrib cross device ops and update all_reduce_alg options. (#6673)
Remove contrib cross device ops and update all_reduce_alg options. (#6673)

* Remove contrib AllReduceCrossDeviceOps and update all_reduce_alg options with MirroredStrategy.

* cleanup

tombstone/models,tombstone/models,derekjchow/models,tombstone/models,alexgorban/models,tombstone/models,tombstone/models,derekjchow/models,alexgorban/models,alexgorban/models,derekjchow/models,tombstone/models,derekjchow/models,alexgorban/models,derekjchow/models,alexgorban/models