add data_generator into paddle.fluid.incubate.data_generator, add op run log in hogwild_device_worker and downpour_device_worker
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dataset_generator.py is a framework for generating data with python
the generated data with a fixed format will be feeded into c++ reader
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add dist_multi_trainer for distributed training, add trainer_factory and device_worker_factory so that we can easily extend new training mode, add pull dense worker which is a singleton for parameter fetching

* fuse optimizer

* Make unitest of fsp op faster and more stable.
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* Skip unitest of fsp op.
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* test=develop, fix space_to_depth_doc

* test=develop, refine indent

* test=develop, refine code

* test=develop, add api spec

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