support specify visible gpus

[jasstionzyf]

[jasstionzyf]

@luckycallor this pull request resolve two things:

1. when not specify pretrained_model in the config file , the train_softmax.py will appear error, i think better way is checking None for pretrained_model parameter.

2. allowed multi training program run at the same time on a multi gpu machine

[dfegdr]

啊打发

[dfegdr]