Features
- Added [ManifoldLoss](https://kevinmusgrave.github.io/pytorch-metric-learning/losses/#manifoldloss). See pull request 635. Thanks domenicoMuscill0!
- Added [P2SGradLoss](https://kevinmusgrave.github.io/pytorch-metric-learning/losses/#p2sgradloss). See pull request 635. Thanks domenicoMuscill0!
- Added the `symmetric` flag to [SelfSupervisedLoss](https://kevinmusgrave.github.io/pytorch-metric-learning/losses/#selfsupervisedloss). If `True`, then the embeddings in both `embeddings` and `ref_emb` are used as anchors. If `False`, then only the embeddings in `embeddings` are used as anchors. The previous behavior was equivalent to `symmetric=False`. Now the default is `symmetric=True`, because this is usually what is done in self supervised papers (e.g. SimCLR).