New features
Add a function of "double_backward" simplifying the training loop (#661)
Bug fixes
Fix issue with setting of param_group for the DPOptimizer wrapper (issue 649) (#660)
Fix issue of DDP optimizer for FGC. The step function incorrectly called "original_optimizer.original_optimizer" (#662)
Replace "opt_einsum.contract" by "torch.einsum"(#663)