Add set_to_none=False for optimizer in workflow

[yiheng-wang-nv]

After Pytorch 1.7, Optimizer adds a parameter set_to_none. When setting to True, it may increase the performance.
Some references:

https://pytorch.org/docs/stable/generated/torch.optim.Optimizer.zero_grad.html

https://pytorch.org/tutorials/recipes/recipes/tuning_guide.html