and optimize the c++ implmentation of empty op as PR#26659 reviews,
and add bool for shape op.

* test=develop, util documents

Replace the 'spawn' start method with 'fork' start method for multiprocessing, on MacOS with python>=3.8 (#27317)

* Replace the 'spawn' start method with 'fork' start method for multiprocessing, on MacOs when python>=3.8

test=develop

enhance reduce op which can reduce tensor with arbitrary rank 

* update model.save_inference_model

* update doc for _save_inference_model, delete useless class in unittests

* make users not be able to set model._inputs be None

* update usage of Model class in unittests

* fix bugs of _verify_spec

* fix bugs of _verify_spec

* add unittest to increase coverage rate

* delete http.log

* update doc for save, remove requirments and limitations for using

* update doc for class Model

* fix floordiv

* Refine the unittest to support py38

    test=develop

Change ProgramTranslator.enable_declarative to ProgramTranslator.enable_to_static to meet API 2.0

* fix fleet dynamic-mode en-doc;test=develop

* add op_function_generator.exe retry in windows, test=develop

* modify test_imperative_using_non_zero_gpu from use two gpus to one gpu, test=develop

* remove objgraph

* remove graphviz

* fix ut

* add adaptivelsgd

* Todo fix the code to avoid the conflict.

* support load state_dict from save_params/persistables

* remove failed unittest

* add load eof check & unittest

* remove eof check

* fix error message

* add input_spec & output_spec for translated_layer

* update error message

* fix ports conflict when launching multi-nodes in paddlecloud;test=develop

* add DISTRIBUTED_TRAINER_ENDPOINTS env for cloud;test=develop

* fix strategy, test=develop

* fix can_apply

* optimize slice TRT plugin

This patch removes unnecessary barrier for data transfer of needed offset,
so data transfer can be overlap with GPU kernel execution.

This patch also fixes incorrect name of slice plugin. That is, replaces
"layernorm" with "slice"

test=develop

* add serialize/deserialize to slice plugin

* add static shape slice trt plugin

* fix slice trt op convertor dynamic shape bug

* fix format by clang-format

* fix pylint format error

* fix problems commented by peiyang
Co-authored-by: NRyan Jeng <rjeng@nvidia.com>

* optimize errror report

* add test case for pad op converter

* fix some spelling mistake commented by peiyang

* replace sequence length attr to input

* Remove the cache in post_traning_quantization, test=develop

* bug fix, test=develop

Fix conv deepwise bug when in_channels=1.

* add check for sparse parameters with weight_decay

* move sparse check to adam.py

* move worker loop to top level

* move reader process loop to top level

* fix failed unittests