* 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

* Add IndicateVarDataType interface to block tensor is not initialized problem in OP GetExceptedKernelType (#20044)

* add indicate_var_data_type inferface, test=develop

* add unittests & polish error message, test=develop

* remove needless include, test=develop

* extract public function & polish message, test=develop

* delete empty var check, test=develop

* change data_type to pointer parameter, test=develop

* polish details, test=develop

* Replace risky GetInputType method with secure IndicateVarDataType interface (#20668)

* replace part of the old implementation, test=develop

* restore concat op, test=develop

* update all ops implemention & delete GetDataTypeOfVar func, test=develop

test=release/1.6

* test=document_fix

* test=release/1.6

* add more err msg, test=develop

* add more unittests, test=release/1.6

test=release/1.6 test=document_fix

test=develop

* 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

* add dlpack include in third_party. test=develop

* test=develop, communicator_is_sgd_optimizer flags

test=release/1.6

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

* improve the performance of capi in PD_PredictorRun (#20665)

* alter the capi of PD_PredictorRun to provide proper function, test=release/1.6

test=release/1.6

* - Flushing mkl-dnn cache

test=develop

- Disabled clearing cache for LoadModel

- Added clearing of mkl-dnn cache when Executor is created

test=develop

- Do not clear for GPU places

test=develop

- compilation fix

test=develop

* - Moved clearing of mkl-dnn cache in destructor of executor

test=develop

* - Compilation fix

test=develop

- Reverted conditional clearing of mkl-dnn cache in Executors's
  destructor

test=develop

- compilation fix

* fix op log bug, test=release/1.6

* add unittests, test=release/1.6

* 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