Update train.py

[JunyaoHu]

solve problem when training.

Traceback (most recent call last):
  File ".../slamp/train.py", line 139, in <module>
    main(opt)
  File ".../slamp/train.py", line 68, in main
    v.train()
AttributeError: 'Namespace' object has no attribute 'train'

the model in origin code contains other dicts like optimizer and epoch, which cannot be set to train mode.