Fix background matting batch size. (#594)
Summary:
Original train batch size: 4 (on synthesized data), 8 (on real data)
The hardware environment is unknown.
Original paper link: https://arxiv.org/pdf/2004.00626.pdf
Pull Request resolved: https://github.com/pytorch/benchmark/pull/594
Reviewed By: aaronenyeshi
Differential Revision: D32729616
Pulled By: xuzhao9
fbshipit-source-id: f3fe69dc86eef0273ab5958796e84a559dac6db7
