* Add seq2seq api related code (#19820)

* fix expand bug  (#20340)

* fix expand bug test=develop

* fix style test=develop

* fix style test=develop

* fix style test=develop

* fix style test=develop

* Fix the assign data check (#20564)

* Fix the assign data check. test=develop

* Fix test_assign_op.py. test=develop

* Update API.spec

Refine error message of transpose. fix en doc about unsqueeze, unstack, transpose, multiplex. test=develop, test=document_preview, test=document_fix (#20594)

* support reshape doubel grad, test=develop

* fix reshape double grad, pass converage, test=develop

* fix review, test=develop

1. fill_constant support Tensor;
2. fix bug in backward.py: using fill_constant instead of fill_constant_batch_size_like
3. fix bug in ExpandGradOp.
test=release/1.6 (#20521)

* Add input check and refine error message

* Refine test case and comments

test=develop
Signed-off-by: Nzhaoyuchen <zhaoyuchen01@baidu.com>

test=develop, test=release/1.6

* Fix the error message of assign op
test=develop

* Refine input type checking for assign op
test=develop

* Refine unittest of assign op
test=develop

 [Cherry-pick #20263] to Add API 'fluid.requird_version(min_version, max_version=None)' to check if the installed PaddlePaddle version is satisfied

please see #20263 for details

* fix error message for elementwise_add/mul;test=develop

* edit error message;test=develop

* add input type and dtype check for accuracy_op

* add input type and dtype check for accuracy_op

* modify python error on accuracy_op,add test=develop

* modify details on accuracy_op, test=develop

* test float16, test=develop

* add warning, test=develop

* add unittest  of seed failed work

* tadd unittest  of seed failed work

* fix the parameter seed failed to work

fix sign op input error check
test=release/1.6

* special case: strategy is None

[cherry-pick] fix Attr(scale)'s bug for image_resize and modified unittest, test=release/1.6 (#19983) (#20460)

[cherry-pick] add input type and dtype check for  elu_op (#20106) fix GradientClipByNorm english doc (#20224) (#20457)

* fix GradientClipByNorm english doc (#20224)

* fix GradientClipByNorm english doc
test=develop
test=document_fix

* add input type and dtype check for  elu_op (#20106)

* elu input check
test=develop

* test=develop

test=release/1.6

* Add fp16 in input.dtype check test=develop

* Add warning of fp16 in CPU test=develop

* add unittest code for fp16 test=develop

* fix float16 list error test=develop

* cherry-pick fix prroi op test=develop test=release/1.6

* 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

Fix the api input type and data dtype check in the op of sign
test=release/1.6

* rm unused ckpt and sort ckpt

* cherry pick test=develop

* cleanup test=develop test=release/1.6

* cleanup test=develop test=release/1.6

* add api check in fc test=develop

* enforce shape error info of sum op test=develop

* fix spelling test=develop

* print x_dims info test=develop

* enhance shape error info test=develop

enhance shape error messages for reshape op.
test=release/1.6

test=release/1.6

* Add support for None for fluid.data (#20228)

* Refine Error Message for New Data API (#20204)

* Modify English doc of fluid.data (#20248)

* Add detailed error messages for nce layer

* Fix some problems

* Fix unit test coverage

test=release/1.6

test=release/1.6

* fix the error message for reduce_mean and reduce_sum op (#20063)

* fix the error message for reduce_mean and reduce_sum op test=develop

* fix typo test=develop

* fix according review advice test=develop

* fix the test test=develop

* fix test=develop

* Fill constant error message fix (#20075)

* fix the constant error message test=develop

* fix typo test=develop

* fix typo test=develop

* fix code style test=develop

* fix comment and bugs test=develop

* fix the bug test=develop

* fix and add unittest test=develop

* fix the typo test=develop

* add support for the fill_constant op test=develop

* add test for ci coverage test=develop

* add error log

add executor.FetchHandler for train/infer from the dataset

* 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, fix auto_prune_error_on_leaf

* test=develop, fix auto prune error on loss stop_gradient

* test=develop, remove useless error check

* test=develop, add more ut for sorted gradient

* Update crf_decoding api & example

* Update api spec

* Fix linear chain crf api

* Avoid sharing data pointer with input

* Simplify the logic in linear_chain_crf_decoding

* Add unittest for crf_decoding when label & path both are set

* Update API spec

* Add unittest for layers && correct infer_shape in chunk_eval

test=release/1.6

test=release/1.6

* Add multihead op for ernie opt

* Refine softmax

* Refine kernel.

* Refine cuda kernel

* Refine cuda version

* Refine cmake