paddle.distributed.fleet supports dynamic graph execution.

* support running python train.py for fleet-task; test=develop

* add FleetAPI doc
Co-authored-by: Nliuyuhui <liuyuhui@baidu.com>

* Support Heter Parameter Server

* fix api document

* add cudnn related strategies to DistributedStrategy

* solve the initial configuration about fleet and rolemaker
Co-authored-by: NseiriosPlus <tangwei12@baidu.com>

* consider the combination of different strategies to work together

* add save/load for parameter server

* set default value to strategy in distributed_optimizer test=develop

* move paddle.fleet to paddle.distributed.fleet

* Directory migration, test=develop

* Change imperative from paddle init to paddle framework, test=develop

* Fixed jit bug, test=develop

* default static mode, test=develop

* fixed format and create parameter belongs to framework, test=develop

* Fixed import package, test=develop

* fix __init__ format, test=develop

* fixed alias problem

* fixed paddle.enable_imperative problems, test=develop

* Add unittest

* delete install_check comment

* Fixed unittest timeout

* fixed unittest error

* move Program default_xx_program to static package

* optimize unittest method

* fixed framework __init__ format

* fixed jit path

* delete alias

* move jit to paddle

* Fixed unittest format

* fixed paddle.default_main_program

* Fixed save load API in paddle __init__.py

* fixed ci paddle.imperative.to_variable

* add paddle.fleet.AsyncOptimizer
Co-authored-by: Ndongdaxiang <dongdaxiang@baidu.com>

* generate context during compile

* split meta optimizer files
* add graph execution in execution, update two properties in DistributedStrategy, unit tests for these features

* refine strategy compiler and meta optimizers
make async as a_sync

* fix gen nccl id bug

refactor fleet api under paddle.fleet
update DistributedStrategy

* add paddle.fleet.DistributedStrategy for 2.0

* Add hapi.text and corresponding unit test.
test=develop

* Remove hapi.text apis' reuse parameter args for coverage.
test=develop

* Fix TransformerCell and TransformerBeamSearchDecoder example codes.
test=develop

* Fix example codes in hapi.text.
test=develop

* Add some apis in hapi.text into example code white list.
test=develop

* Fix example code of DynamicDecode in hapi.text.
text=develop

* Rename Model.self as model in test_text.py
test=develop

* fix high level API(Inference) bug

* patch the unit tests

* Use for_test=True in the Fluid Trainer to clone the test program

* fix typo

* Should do the samething to the inferencer

* add Inference.infer

* optimize code

* update no_test_word2vec_new_api.py

* update trainer

* split check_and_get_place

* use inference_program to save inference model in Trainer

* update demo

* update save_inference_model

* clean code

* Load/save the params from the params_path

* Switch to use load_persistables and save_persistables

* Instaed of setup the executor to run program and scope. Pass the program to the load_persistables

- The trainer and inferencer will load params from disk if param_path
  argument is not None in their constructor.

- Remove params.py, we will expose core.Scope to the user if needed
  (e.g., for GAN). Currently we will not expose it, unless we clearly
  know doing so can support GAN.

- Add `save_params` to Trainer (a TODO item).

- rename "network" to "program"