Add CPU implementation to pytorch_CycleGAN_and_pix2pix (#790)
Summary:
Note that the CPU run is very slow so it is disabled in nightly run by metadata (https://github.com/pytorch/benchmark/blob/main/torchbenchmark/models/pytorch_CycleGAN_and_pix2pix/metadata.yaml#L10).
Still, users can run CPU train or eval test with the `run.py` command:
```
$ python run.py pytorch_CycleGAN_and_pix2pix -d cpu -t [train|eval]
```
Fixes https://github.com/pytorch/benchmark/issues/788
Pull Request resolved: https://github.com/pytorch/benchmark/pull/790
Reviewed By: anijain2305
Differential Revision: D34832041
Pulled By: xuzhao9
fbshipit-source-id: 111622206fc82defa4641bcf03d82740e035bd01
