Assign placement on operator
Created by: Yancey1989
tasks:
- design doc: https://github.com/PaddlePaddle/FluidDoc/pull/107
- Python API to assign place on operator: #13649
- refine startup_program so that initializing the var on the valid place.
- Using a pass to insert the communicator (memcpy, send, listen_and_serv) at the boundary between ops which on the different place.