Created by: chenwhql

The dygraph parallel interface Env and DataParallel not public to users.

The semantics of the old class names are inaccurate and may confuse user, so sthis PR rename the Env class to ParallelEnv, add compatible implementation, and add documentation for ParallelEnv.

Chinese doc: https://github.com/PaddlePaddle/FluidDoc/pull/1892