Add nanogpt model (#1628)
Summary:
This adds nanogpt model to torchbench. Only adds a NLP.Geneartion task so no training script
Fixes https://github.com/pytorch/benchmark/issues/1608
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1628
Reviewed By: xuzhao9
Differential Revision: D45778409
Pulled By: drisspg
fbshipit-source-id: 83f9e81fdded31261f123f6600f2a069145ddb24
