* support backward return None.

* edit unittest.

* edit code according to CI

* Improve error information

* optimize slice op and slice grad op, test=develop

* optimize variable name and annotation information, test=develop

* skip op has no fp16 kernel

* add ut

* add norm_by_times param to ctc_loss

* fix doc,test=develop

paddle.to_tensor will be translated to paddle.assign in Dy2stat, however paddle.assign doesn't support int, float, bool. This PR added the supports.

* [DOC] Clarify the difference between paddle.norm and np.linalg.norm

* add pp dygraph, test=develop

* Dygraph reocmpute

* unitest for Dygraph reocmpute

* dy recompute remove unitest for win and mac

`range` API set its output shape in dygraph but not in static graph, which can cause Dy2stat error. This PR set the shape of `range` API when possible.

* fix airank bert emb order

* move input num check to converter

* add input num check

* add unused var check white list

* update testcases

* remove unused codes

* update the docstring for samcd_processor

* no need to import the six module

* 我也不知道为何有一个前导空格，但现在有单元测试，取消这个空格没啥问题

* add unittests for print_signatures; add the first case for 'required' mechanism when executing sample code testing

* there is no paddle installed in PR-CI-APPROVAL

test=document_fix

* support python39 and delete python35

* support python39 in Dockerfile.centos

* fix ubuntu18 bug

* update Dockerfile.ubuntu setuptools

* fix centos py39 errors

* fix centos py39 error2

* add trt runtime version check

* use different wrap, and change to major version check

* support save/load binary format tensor

* Fix error when create cudaplace

* Fix error when create cudaplace

* Fix error when create cudaplace

* get devive context from pool.

* move define of 'SerializeToStream' and 'DeserializeFromStream' to 'lod_tensor.cc' and 'selected_rows.cc'.

* support complex object

* improve coverage.

* improve coverage

* improve coverage.

* fix a bug.

* polish API

* save/load program

* paddle.save/load: layer

* deal with conflict

* if PY2, block test_paddle_save_load.TestSaveLoadLayer

* polish code.

* polish code

* edit unnittest

* The condition for object to be identified as state_dict becomes strict

* use 'core._cuda_synchronize'

* add Hub Module for easy to use pre-trained models.
*   support list, load, help fucntions.
*   support load models by github, gitee, local 
Co-authored-by: NLielinJiang <jianglielin@baidu.com>

* add pipeline layer

* update

* fix bug: when x.dim < y.dim, the result of compare_op is inverse to expected result

* support the cuda for fix the compare broadcast bug

* fix tc trt shape

* fix fc dynamic shape

* add fc shape assert

* update

* let paddle.utils.install_check support CPU package with GPU device

* use use_cuda in dygraph checking

* add unittest for install_check