* 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

checkpoint for distributed training.

* fix io doc
test=develop

* refine data_feeder doc
test=develop

test=develop

*  fix mkdir conflict

*  fix load/save lookup tables

 test=develop

* add lookup_table_utils

* fix load optimize vars on pserver

* delete lookup table utils

* fix save and load lookup tables

* fix load optimizer var

* fix load optimizer var, test=develop

* fix python 3 style, test=develop

* move lookup_table_utils to contrib utils

*  fix mkdir conflict

test=develop

* fix dim0 in _load_slice_up_vars

* fix dim0 in _load_slice_up_vars, fix innershape in delete_var_op

* Revert "fix lookuptable in reduce strategy"

This reverts commit 0e722c5e

* add unit test for dist

* add unit test for dist, test=develop

* cancel revert, test=develop

In the future, we'd like to encourage user to save
everything during training.
This allows us to
1. Do more flexible optimization passes
2. Re-train and fune-tune

* hide program APIs

* fix merge error

* update

add export_for_deployment flag to save_inference_model