add LambdaRank DCG Loss Option (#23679)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23679
Full Canary: https://fburl.com/fblearner/sa1pkpya
Add LambdaRank DCG Loss Option
* when use_idcg_normalization == true, regular LambdaRank with NDCG loss
* when use_idcg_normalization == false, gradient and loss functions are not normalized by idcg.
Differential Revision: D16605459
fbshipit-source-id: a16f071e69516974e48d27bef4ca179019ca4ae7
