* Serialize to pickle format (#20820)

test=develop

* save load problem fix and new feature add (#20823)

* fix persistable;

* fix save load bugs; test=develop

* fix bug; test=develop

* add example for new io api; test=develop

* addd example; test=develop

* test=document_fix

* test=release/1.6

test=release/1.6 test=document_fix

* add support for __get_item__ of Variable test=develop

* update the infer shape of matmul,  test=release/1.6

* add unittests of matmul, test=release/1.6

* change func names, test=release/1.6

* test=develop, communicator_is_sgd_optimizer flags

【cherry-pick】 fix fill_constant shape with -1 and enhance cross_entropy test=release/1.6 (#20722) (#20728)

* fix fetch handler error with pslib

* fix distributed lookup table op with 1 pserver

* Fix communicator slow bug & fix communicator stop bug (#20366)

* test=develop,Fix communicator slow bug

* test=develop, delete if() in stop_worker()

* test=develop

* fix UT, test=develop

* fix bug in fetch handler, test=develop

* fix bug in fetch handler, test=develop

* test=develop, fix fetch barrier bug

* test=develop, bug fix

* test=develop, bug fix

* test=develop, fix bug

* test=develop,test=release/1.6

* move pool2d to add_quant_dequant_pass, test=develop

test=release/1.6, test=develop

* Support fp16 in fused_elemwise_activation_op.
* Fix unit testing in ONLY-CPU mode.

test=release/1.6

* bug fix: invalid learning rate decay in pserver async mode

* support dump multi file
test=develop

* dump fix num file
test=develop

test=release/1.6

* cherry-pick refine slim reader to support dataloader test=release/1.6

* cherry-pick add test for reader test=release/1.6

* cherry-pick rm checkpoint path test=release/1.6

* cherry-pick fix details test=release/1.6

* cherry-pick fix details test=release/1.6

* disable nccl test

* Update version.

* fix term core only

* fix transpiler error

* fix protobuf memory leak (#11177)

fix protobuf memory leak

* "change eigen mirror"

* test=document_fix

* test=document_fix

* add input type and dtype check, enhance shape error message for concat_op (#20101)

* add input type and dtype check, enhance shape error message for concat_op
test=develop

* enhance shape check
test=develop

* improve coverage

test=develop

* enhance input type chec for concat (#20584)

test=develop

* add expand_as op, test=develop

* add expand_as op,test=develop

* add expand_as op,test=develop

* add nn.py, test=develop

* delele paddle_enforce, test=develop

* add data type check, test=develop

* polish error messages, test=develop

* polish error messages, test=develop

* Remove support for the CPU architecture matmul, test=develop

* fix syntax bug, test=develop

* modified group_norm's unittest for pass statement, test=develop

* fix group_norm's bug: scale or bias is None which causes segmentation fault, test=develop

* fix recurrent bug
test=develop

* Fix while op bug
test=release/1.6

* add unit test
test=release/1.6

* Fix docs of gru_unit and dynamic_gru.

Fix basic_gru in rnn_impl.py.

Add error messages for param_attr setting in layer_norm api.

Add int64 dtype for expand.

test=develop

* Reopen unit-tests of basic_gru/basic_lstm in rnn_impl.py.
test=develop

* Add unit test for layer_norm api.
test=develop

* Remove the deprecated gru doc fix. test=develop

* Fix basic_gru test coverage. test=develop

* Update API.spec. test=develop

* Update API.spec. test=develop

* Fix test_basic_gru coverage test. test=develop

* Update test_basic_gru in test_layers to use fluid.data
test=develop

* Update test_basic_gru for coverage. test=develop

test=release/1.6

test=release/1.6

* fix the error message of zeros op
test=develop

* Fix unittest of zeros op
test=develop

* 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)