Update opacus_cifar10 to latest Opacus API (#681)
Summary:
Fixes https://github.com/pytorch/benchmark/issues/604
Pull Request resolved: https://github.com/pytorch/benchmark/pull/681
Test Plan:
```
python test.py -k test_opacus_cifar10_train_cpu
python test.py -k test_opacus_cifar10_eval_cpu
python test.py -k test_opacus_cifar10_example_cpu
```
Reviewed By: xuzhao9
Differential Revision: D33404993
Pulled By: zou3519
fbshipit-source-id: 7181682be375d9bbf2bac12bebb1f27e72022600
