Created by: TomorrowIsAnOtherDay

This PR:

• add parl.compile.
• replace PE with compiler in A2C/IMPALA.

learn_program = fluid.Program()
....define the learn_program and cost 

learn_program = parl.compile(learn_program, cost.name)
agent.fluid_executor.run(learn_program)

I haven't used this API for GA3C/competition directory as it may cause thread-safe problems when using parl.executor in different threads.