auto data sync between different devices
Created by: jacquesqiao
project: https://github.com/PaddlePaddle/Paddle/issues/6403 In a sequence of operators, when some of them run on GPU, some can only run on CPU, we need to auto add some operators to copy host memory and device memory, so the whole graph can run in a multi devices environment.
some problem
- now we use name to get var in scope, and data on different devices should have different names, if we add some operator to sync data between different devices, we should change the input name of corresponding operators.