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.

Error description optimize for the math dir

fix judge cache file of inference api more accurate

test=develop

* support load state_dict from save_params/persistables

* remove failed unittest

* add load eof check & unittest

* remove eof check

* fix error message

* polish framework error message part 7

* fix typo

* polish by reviewes comment

* add input_spec & output_spec for translated_layer

* update error message

update the error message check for the some ops

add the error message check for the some operator

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

* add DISTRIBUTED_TRAINER_ENDPOINTS env for cloud;test=develop