Added Nesterov momentum to SGD optimizer

[georgekasa]

[georgekasa]

I added a """feature"""" if someone could add Nesterov momentum in SGD opt, train_ssd.py -> line 375 optimizer = torch.optim.SGD(params, lr=args.lr, momentum=args.momentum, weight_decay=args.weight_decay, nesterov = True)

as Karpathy told in CS231n: Nesterov Momentum is a slightly different version of the momentum update that has recently been gaining popularity. It enjoys stronger theoretical converge guarantees for convex functions and in practice it also consistenly works slightly better than standard momentum.