Fix the train batch size of densenet121 (#579)
Summary:
Use the batch size of 256 from the densenet121 paper:
Source: https://arxiv.org/pdf/1608.06993.pdf
Pull Request resolved: https://github.com/pytorch/benchmark/pull/579
Reviewed By: xuzhao9
Differential Revision: D32650565
Pulled By: aaronenyeshi
fbshipit-source-id: 781f1afb4fad167c43a161f8e8f97ff24b00a9a5
