Created by: chengduoZH

fix https://github.com/PaddlePaddle/Paddle/issues/4637 Add pool cudnn op.

• Finish the cudnn pool2d maxpool and avgpool, only support NCHW(data layout) .
• Finish the cudnn pool2d unit test.
• Fix pool_op attribution's name.
• TODO in next PR:
  • Add cudnn pool3d maxpool and avgpool.
  • Add cudnn pool3d unit test.
  • Support NHWC(data layout) .