add an option to save checkpoints at a specific epoch

[zydou]

The best checkpoints rarely occurred in the first few epochs. So adding this option will reduce the trainning time. For example:

python examples/triplet_loss.py -d market1501 -a resnet50 --combine-trainval --logs-dir examples/logs/triplet-loss/market1501-resnet50

and

python examples/triplet_loss.py --start_save 130 -d market1501 -a resnet50 --combine-trainval --logs-dir examples/logs/triplet-loss/market1501-resnet50

gives the same results. But the latter is faster.