Add Similarity Net as unit test. During the unit test, we found three problems:

1. The run_program_op has memory optimization error when running dy2stat net multiple times.
2. The support for SelectedRows can cause problem in dy2stat.
3. The return grammar has problem.

This PR fixes the 1. problem but modify codes for the 2. 3. problems to make PR smaller. I will fix those two problems in the next PR(s)

* Add the imperative quantization aware training.
*  This is the python part of Imperative QAT. test=develop

* fix optimizer.state_dict and LRScheduler.state_dict to save/load dygraph,test=develop

* fix optimizer.state_dict and LRScheduler.state_dict to save/load dygraph,test=develop

* Add a judgment that state_dict/set_dict is used incorrectly,test=develop

* fix some doc error,test=develop

* fix current_step_lr for _LearningRateEpochDecay,test=develop

* remove some unsed code to improve coverage,test=develop

* remove some unsed code to improve coverage,test=develop

add new API:LambdaDecay

* attempt to resolve tls problem, test=develop

* add glibc version check, test=develop

* fix regex, test=develop

* refine get_libc_ver, test=develop

* refine get_libc_ver, test=develop

* fix optimizer parameter is a iterator; test=develop

* fix parameter list None bug; test=develop

* use is not None; test=develop

* change list to iterable; test=develop

* refine as_lodtensor, test=develop

* fix test, test=develop

* add unittest, test=develop

* handle nested_list, test=develop

* handle nested_list, test=develop

* add retry for prefetch

* Revert "Fix/sync barrier (#25016)"

This reverts commit be6a315f.

* reopen dist UT, test=develop

* remove fl UT, test=develop

test=develop

* fix test_multiprocess_dataloader_exception failed on CPU only version. test=develop

* clean __str__ of VarBase and ParamBase, test=develop

* clean to_string, test=develop

* update unittest, test=develop

* fix bn & in in dy, test=develop

* update instance_norm,test=develop

* fix bugs,test=develop

* add more case in unittest,test=develop

* fix,test=develop

* fix,test=develop

* add device_worker for pipeline, test=develop

As the title

As the title

fix nce sample code for python3 compatability.

* connect stop_gradient with trainable; test=develop

* add value error message; test=develop

* add paddle.fleet.DistributedStrategy for 2.0

* add Pad2D and Leaky_ReLU, test=develop

* update,test=develop

* change name,test=develop

* add unittest and redine docs,test=develop