* support save/load single tensor

* compatibility modification according to unnittest

* Some python2.7 don't have 'copyreg' modules

* Handle a syntax error.

* Dealing with compatibility problems on Mac.

* Dealing with compatibility problems on Mac.

* edit unittest to improve coverage.

* Modify the code according to the review comments

* Reduce redundant code.

* support for static graph loading dygraph state_dict

* edit code according to CI

* edit unittest

* edit unnittest

* delete redundant file

* edit code according to Comments

* edit english doc

* edit english doc

* edit English DOC.

* get/set_tensor->get/set_value; return_numpy=False

* get/set_tensor->get/set_value; return_numpy=False

* edit unnittest

* edit unnittest

* polish code.

* add default argument  for paddle.save/static.save

* edit documentation of

* Add comments for special processing for protocol=2 and protocol=3.

* Update python/paddle/fluid/io.py
Co-authored-by: Nlanxianghit <47554610+lanxianghit@users.noreply.github.com>
Co-authored-by: Nlanxianghit <47554610+lanxianghit@users.noreply.github.com>

* Avoid  bug on 'MAC python3.5/6'.

* Choose the saving method according to the OS.

* smaller length of '_unpack_saved_dict' for MAC OS.

* add version information of Python.

* Edit comment.

* Support storage of large parameters

* Reduce the complexity of the unittest

* Reduce the complexity of the unittest,commented out unittest for

* add unittest for static.save/load

* Increase the timeout threshold of 'test_static_save_load'

* Increase the timeout threshold of 'test_static_save_load'

* Increase the timeout threshold of 'test_static_save_load' and 'test_paddle_save_load'

* Increase the timeout threshold of 'test_static_save_load' and 'test_paddle_save_load'

* make Numpy version is below 1.19.3

* fix 2.0 doc

* polish save load en doc details, test=document_fix

* polish details, test=document_fix

* remove blank, test=document_fix

* fix ascii error, test=document_fix

* update fc's input to x, test=document_fix

* update fc argument activation, test=document_fix

* fix doc and unittest of 2.0 lr_scheduler

* fix doc of 2.0 lr_scheduler

* fix unittest

* fix english doc of lr_scheduler

* fix api name of lr scheduler

* fix api name of lr scheduler

* replace config by kwargs

* change save path form dir to prefix

* fix failed unittests

* revert unittest name change

* polish en docs

* add more tests for coverage

* init commit of new save/load

* fix failed unittests

* fix save_load_v2 unittest failed

* fix failed unittest & polish doc

* add tests for coverage

* add more tests & move static apis

* fix example code error

* polish emample code

* fix detail example code problem