Created by: wanghaoshuang

Conv layer will check 'use_gpu' option while layer_type is setted to cudnn_conv at code. But conv layer can't can get use_gpu option correctly at here. I guess the solution of parsing use_gpu from cmd line is not compatible with v2 api. We can fix this bug by:

1. Remove check use_gpu
2. Put use_gpu option into os.environ.iteritems() while paddle.init() is called.
3. Or other better solution?

And another bug: cudnn_convt should be added here for supporting trans_conv in gpu mode.