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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* test=develop, add install check

* test=develop, add install check scripts

* test=develop, refine language

* test=develop, add api spec

* test=develop, change cdn to bj to pass ci

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* extend the slice function for python
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* test=develop, add some Layers and tests

* test=develop, add more layers

* test=develop, add more layers

* test=develop, add force cpu option

* Update test_layers.py

remove pdb

* test=develop, refine code

* fix cdn issue, test=develop

* fix memory optimize bugs, test=develop

* fix memory optimize bugs, test=develop

* remove add/sub_2 op, test=develop

* disable memory_optimize by default, test=develop

* disable inplace activation in python, test=develop

* fix unittests, test=develop

* fix unittests, test=develop

* bug-fix, test=develop