Multi-GPU support

[jacquesqiao]

see below in detail:
Survey on Multi GPU Feature https://github.com/dzhwinter/Paddle/wiki/Survey-on-Multi-GPU-Feature

[dzhwinter]

1. Survey different frameworks design style before 8/25(Friday) EOD
   • tensorflow
   • caffe2
   • NCCL library
2. design doc of the interfaces between GPU cards, parameter server.

[dzhwinter]

Survey on Multi GPU Feature https://github.com/dzhwinter/Paddle/wiki/Survey-on-Multi-GPU-Feature