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

* add event for fast executor and add threads for scopebuffer executor
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

* fix bug for ctr_reader test=develop

* fix svm data test=develop

fix svm data 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

Optimize recurrent_op using Prepare and RunPreparedContext, avoiding create operators in every iter. (#17689)

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

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

test=develop

Optimize the concat and split kernel for specical cases when the number of inputs/outputs is 2 (#17415)

* Optimize the concat and split kernel for special cases that the number of inputs/outputs is 2.
test=develop

* Refine codes.
test=develop

* Correct the condition.
test=develop

* Move the define of tmp_data outside the if statement.

* Print the cudnn minor version.
test=develop

* Fix the case when in_num/o_num is 1 in concat/split op.
test=develop

* Remove const_cast.
test=develop

* 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