* enhance load interface; test=develop

* add uni test and add comment; test=develop

* fix converage; test=develop

* use path.joint replace "/"; test=develop

* windows debug; test=develop

* fix window unitest error; test=develop

* fix commet error; test=develop

* add model shuffix check; test=develop

* fix example error; test=develop

* copy stop_gradient property in clone, test=develop

* fix pruning issue, test=develop

* fix kLookupTablePath issue, test=develop

* sync program, test=develop

* copy subblocks, test=develop

* add unittests, test=develop

* add fake init for the trainer, fix large memory hold in the trainer
* do not merge recv vars from a remote endpoint, test=develop
* add recv and save op, merge slice var in one op, save memory
* remove hsigmoid with pull sparse, test=develop

* fix persistable;

* fix save load bugs; test=develop

* fix bug; test=develop

* add example for new io api; test=develop

* addd example; test=develop

test=develop

Modify doc for shuffle, firstn, save_vars, load_vars, L1DecayRegularizer, L2DecayRegularizer (#20287)

* modify shuffle, firstn, regularizer, load_vars, save_vars, test=develop, test=document_fix

* add new save load interface; test=develop

* add new save interface; test=develop

* add save load interface ;

* fix save load error;

* fix dygraph set dict bug;

* add save load unit test; test=develop

* fix test_imperative_optimizer bug; test=develop

* fix unitest optimizer bug; test=develop

* fix code coverage; test=develop

* fix converage; test=develop

* add document for apis; test=develop

* fix unitest error; test=develop

* fix save load unit test error; test=develop

* fix error message; test=develop

* change set_parameter set_optimizer to save_dygraph; test=develop

* add load_graph check; test=develop

* fix api spec; test=develop

* improve the docs of load_inference_model&load_params&load_persistables&chain.

* add a default value for _prune interface

* modify document

* add parameter shape check when load parameter from file; test=develop

* fix test error; test=develop

* add wrong shape check; test=develop

* remove useless code; test=develop

* add testcase setup

* add teardown, remove temp model_path; test=develop

* add clean process; test=develop

* update operator compatible info, test=develop

* revert cmake/version.cmake, test=develop

* add unit_tests and fix bugs, test=develop

* update ../paddle/fluid/framework/framework.proto, test=develop

* fix bug of paddle/fluid/inference/api/analysis_predictor.cc, test=develop

* update paddle/fluid/framework/version_test.cc, test=develop

* add comments and rename interfaces, test=develop

* give warnings when save a model without any parameters test=develop

* delete one line comment test=develop

* Fix bug: add feed_vars to the prune function

* use PE for compiler
test=develop

* add check of executor, test=develop

1. fix the bug that out_put_var in SaveSelectedRows would be empty string
2. use merge_sparse_lookup_table to replace sum op for load_persistables_for_inference
3. fix the bug in _clone_var_in_block_ when the var is SELECTED_ROWS.

The fixed APIs:
6 Methods in paddle.fluid.io.PyReader
paddle.fluid.layers.Preprocessor
paddle.fluid.layers.py_reader
paddle.fluid.io.save_params
paddle.fluid.io.save_persistables

test=develop
test=document_preview

* fix save/load in Fleet
* add UT framework of Fleet

* fix logging unable. test=develop

* unset sys.stdout for stream handler. test=develop

* fix newly add basicConfig. test=develop

* fix import error. test=develop

* fix the bug that sub_scope_ may be null in AnalysisPredictor::Run.

* add more directions about io APIs' docs.

* update the API.spec. test=develop test=document_preview

Fix some api example codes' bugs and these APIs include load_inference_model, load_vars, save_vars, L1DecayRegularizer and L2DecayRegularizer. (#17324)

* fix some api example codes' bugs.

* update API.spec. test=develop test=document_preview

* add import fluid. test=develop test=document_preview

The parameter dirpath will be passed directly to c++ operater. The file address format will be different under win and UNIX.

* Fix API example code of save_inference_model

test=develop

* Add "import" in exmaple of save_inference_model

* Fix typo "exsample" -> "example"

test=develop

* save infer model bug fix, return target vars' name list

* fix params with only 1 dim
* test=develop

test=develop