* Add c++ global current tracer for dygraph, test=develop

* add tracer property in c++, test=develop

* support different place, test=develop

* add unittest for tracer, test=develop

* don't expose numerous Tensor.set(), test=develop

* fix condition, test=develop

* fix float16 bug, test=develop

* feed should be Tensor or np.array, not Variable or number, test=develop

* use forcecast to copy numpy slice to new array, test=develop

* remove float16-uint16 hacking, test=develop

* jit/trace 1st version, test=develop

* add more unittests, test=develop

* fix en docs of Layer and guard
test=document_fix, test=develop

* fix en docs of Layer and guard
test=document_fix, test=develop

* minor fix
test=document_fix, test=develop

* minor fix
test=document_fix, test=develop

* fix api.spec
test=document_fix, test=develop

* fix api.spec
test=document_fix, test=develop

* fix docs
test=document_fix, test=develop

* fix docs
test=document_fix, test=develop

* fix docs
test=document_fix, test=develop

* fix api.spec
test=document_fix, test=develop

* fix api.spec
test=document_fix, test=develop

* add forward doc
test=document_fix, test=develop

* add "s" for parameters
test=document_fix, test=develop

fix APIs, to_variable、NCE、PRelu、softmax、rankloss for dygraph, test=document_fix, test=develop (#20142)

* test=develop, fix docker with paddle nccl problem

* test=develop, Add Variable api and refine dygraph related API

* test=develop, Add Variable api and refine dygraph related API

* test=develop, refine test for new api and error info

* test=develop, refine error info and test_layers

* test=develop, add API.spec

* test=devleop, fix to_string python2 and python3 compat error and refien doc

* test=devleop, add API spec

* test=devleop, update API spec

* test=devleop, update API spec

* test=develop, invoke ci

* test=develop, fix example code

* test=develop, update API spec

* test=develop, add compat test and fix inplace campat dict error

* Add fp16 support for dygraph
test=develop

* Add unit test
test=develop

* update dygraph api-doc and backward api-doc, test=develop

* update dygraph api-doc and backward api-doc, update api.spec, test=develop

* update dygraph api-doc and backward api-doc, update api.spec, test=develop

* update API.spec, test=develop

* refactor dygraph,test=develop

* fix failed unittest,test=develop

* polish code,test=develop

* check windows ci error,test=develop
try to fix windows ci error by np.allclose,test=develop

* polish vlog and profiler, test=develop

* try to fix preceding ops order,test=develop

* test transformer in windows ci, test=develop

* use python c-api to speed up tracer.trace,test=develop

* test=develop, fix docker with paddle nccl problem

* test=develop, add ut for debug string and gradient_accumulator

* test=develop, add tests for layer/gradient_accumulator/prepared_op

* test=develop, fix complie error for test_prepared_op

* test=develop, add more ut for dygraph

* test=develop, create API.spec for dygraph api change

* test=develop, refoctor name to make it easier to understand

* test=develop, refoctor name to make it easier to understand

* test=develop, fix multi-gpu failed problem , add Tracer tests, change PADDLEENFORCE to PADDLEENFORCE_EQ

* test=develop, fix ut failed on parallel se-resnext

* test=develop, change one more PADDLE_ENFORCE

remove dygraph.enable from __all__
hidden dygraph. profiler
add doc to dygraph. no_grad

* test=develop, fix docker with paddle nccl problem

* test=develop, hide no_support api and add ut for it

* fix dygraph mem leak, test=develop

* polish msg, test=develop

* WIP

* WIP

* test=develop, add api doc and example code for dygraph

* test=develop, add dygraph_not_support and refine ocr

* test=develop, shrink name of dygraph_not_support

* save optimizer related vars in dygraph

* test=develop, add optimizer save and load

* test=develop, add optimizer save and load

* test=develop, merge code and add multi-optimizer save and load

* test=develop, fix test_imperative_checkpoint

* test=develop, fix include error

* test=develop, fix include error

* test=develop, renew api spec

* test=develop, refine code

* test=develop, set default value for checkpoint

* test=develop, fix ci error

* test=develop, change API.spec and make api more readable

* test=develop, refine version and time stamp

* test=develop, add example code and refine code

* test=develop, refine doc

* test=develop, change version

* add no_grad decorator to dygraph, test=develop

* add unittest,test=develop

* add layer norm to Layers, add transformer prepare encoding

* little change

* finish encoder part

* add decoder part

* finish model part

* add test case and part of data feed

* add transformer test

* add to_parameter, add remove in set_attr

* test=develop, fix pos encoding bug, create_parameter with stantard name

* test=develop, rm dropout test in imperative

* test=develop, fix cpu error

* test=develop, fix minize bug

* test=develop, fix one hot not stop gradient

* test=develop, fix one hot not stop gradient

* test=develop, refine parameter name

* test=develop, fix transformer test in imperative mode

* test=develop, fix transformer test in imperative mode

* test=develop, fix boost and mkl download error

* test=develop, fix boost and mkl download error

* test=develop, fix ci and refine code

* test=develop, fix ci and refine code

test=develop

test=develop

test=develop

test=develop

test=develop

test=develop

test=develop

Revert "Merge pull request #14798 from PaddlePaddle/revert-14786-revert-14782-revert-14398-imperative"

This reverts commit b1d3a1c8, reversing
changes made to f1fb64b1.

test=develop

test=develop