test=develop

* Enable elementwise_div operator test=develop

* Fix update date test=develop

Note the append_batch_size variable is doing prepend. We should
change the name, but due to backward compatibility, I suggest to
change at v2.0. Not now.

test=develop

* add no_grad decorator to dygraph, test=develop

* add unittest,test=develop

 use numpy assignment to restore parameters at build-once

* fix the dll not found issue on windows

* add example to get_startup_program()
* fix example to get_startup_program()

Split the unittest test_dist_mmist into multiple unittests (test_dist_mnist, test_dist_mnist_nccl and test_dist_mnist_lars) to avoid timeout (#17707)

* test=develop modify some initializer api

* test=develop modify API.spec

* test=develop modify API.spec

* test=develop modify API.spec

* test=develop modify API.spec

* fix prepare context redundant code problem, optimize executor by caching create_varaiables
test=develop

* cache sub_scope, program, var when use_program_cache=True is set

* make fetch_list runable with variables, add more unittest for use_program_cache

test=develop

* fix API examples of assign, reverse and array_write
test=develop

* update API.spec
test=develop

* update API examples for array_length, array_read, array_write, assign, hard_sigmoid, hsigmoid, increment, ones, pow, reverse, uniform_random and zeros

* update API.spec for assign, reverse and array_write, etc.(#17287)
* test=develop

* unit commits, test=develop

* update API.spec, test=develop

* API.spec test=develop

* update

* update test=develop

* update test=develop

* update

* update test=develop

* update test=develop

* update test=develop

* update test=develop

* update test=develop

* test=develop

* update

* update test=develop

* update test=develop

* Enhance fused_elementwise_activation op.
test=develop

* Move the api fused_elementwise_activation to contrib.
test=develop

* Add including files.
test=develop

* Add the support of sigmoid in fused_elementwise_activetion op.

* Update API.spec.
test=develop

* Enable less_than ngraph operator

test=develop

* Added compare unit-tests test=develop

* Update: date && removed import test=develop

* fix:
1. infernce multi card occupy
2. facebox model inference occupy too much
test=develop

* refine python api comments: shuffle, while, scale, sampled_softmax_with_cross_entropy, scatter,
round, sin, sqrt, shape, split, soft_relu, slice, selu, ifelse, switch.
test=develodp

* fix conflict error.
test=develop

* test=develop, add ocr in dygraph test

* test=develop, add cudnn determinist

* test=develop, remove useless code

* test=develop, fix cmake error

test=develop

* add depthwise_conv2d test=develop

* use cpu for ngraph test=develop

fix im2sequence padding bug

* fix doc in transpiler, test=develop

* add gradient clip in minimize; test=develop

* fix bug; test=develop

* fix format; test=develop

* move new grad clip to dygraph/grad_clip.py; test=develop

* fix lr decay and grad clip test; test=develop

* seperate dygraph grad clip; test=develop

* fix grad clip test; develop

* fix api spec bug; test=develop

* add blank line, test=develop,test=document_preview

to fix format problem

* update python APIs

test=document_preview test=develop

* update API.spec

test=document_preview

test=develop

* update merge_selected_rows

* update API.spec

test=document_preview

test=develop

* update API.spec

test=document_preview

test=develop

* fix the comment of less_than

test=develop

test=document_preview

* update API.spec

test=develop

test=document_preview

* update API.spec

test=develop

test=document_preview

* update API.spec

test=develop

test=document_preview

* update API.spec

test=develop

* update API

test=develop

test=develop

* [DOC] Fix api docs, test=develop, test=document_preview

* [DOC] Fix api annotation: fluid.layers.tensor_array_to_tensor. test=develop, test=document_preview

* test=develop, test=document_preview

update MD5 of tensor_array_to_tensor