fix initial_lr is not specified in param_groups[0] when reuming

[LiUzHiAn]

Hi,

When I tried to resume the training process, I was told that 'initial_lr' is not specified in param_groups[0] when resuming an optimizer. So, I guess we have to feed this key to the optimizer. That is:

optimizer = torch.optim.SGD([{"params":model.parameters(),"initial_lr":args.lr}], lr=args.lr,
                                momentum=args.momentum,
                                weight_decay=args.weight_decay)