* Adding communication part in readme, test=develop

* modify_readme_contents,test=develop

* Update README_cn.md

* test=develop
support Tensor input for chunk_eval op

* test=develop
fix testcase for chunk_eval op

* test=develop
fix typos in nn.py

move the thread-check code from train_from_dataset to a single function
add UT for the thread check function

* rename mkldnn set/get_cur_thread_id() to set/get_cur_mkldnn_session_id()

test=develop

* update session id definition and adjust logic for default behavior

test=develop

* reset logic in mkldnn reuse as most of cases work in default.

test=develop

test=develop

1. Since allreduce op has 4 reduce types, We split these four reduce types into four ops
2. We also refined the collective op code, e.g. we separated the collective op kernel into CPUKernel and CUDAKernel, and remove the device specified DeviceContext parameter in template as we already knew the target DeviceContext
3. We remove the newly added Collective op role to reduce the complexity of program and graph analysis

* fix the api.spec file does not get the class comment problem

* cat new.spec

* check api.spec

* test=develop

make fleet support mpi job submit directly.

* add not_been_used_vars to no_grad_set
test=develop

* modify roi_perspective_transform_op to output mask and transform matrix

* modify comment

* modify comment

* modify API.spec

* update API.spec

* remove no use header, test=develop

* resolve conflict

update README_cn.md with latest version

update README.md with latest release & install version

* fix mac ci random fail
* use platform instead

* Int8: Fix Pooling output scale

test=develop

* Update scales quantization for certain operators

These include: concat, transpose, pool and reshape. test=develop

* Move concat minimum scale finding to quantizer

test=develop

* Fix bug in quantize kernel which cause crash in vgg16/19 model

test=develop

* refine the code to reduce verbose code; test=develop

* remove useless code; test=develop

replace mnist dataset url

* update api format
test=develop

* update API.spec
test=develop

fix data feed ptr runtime error, pipeline trainer will core in some cases, so set it nullptr as default value.

change url of pslib.tar.gz

* fix py-cpuinfo mac random fail
* differentiate version on windows

test=develop

* test=develop

* test=develop

* add MultiSlotStringDataGenerator for speedup of string based user input data

1. some key generation method is not aligned with PR#17965
2. enlarge ptr lifetime to avoid memory release if SetBlob fails
   otherwise it will get core dump.

test=develop

* change nproc 8

* add_elementwise_add_inplace_test,test=develop

* rename file, test=develop

* test=develop, add some comments for Program.clone

* test=develop, add API.spec

* test=develop, refine comments

* refine Program doc and clone doc

* test=develop, refine doc

*  add cuda_is_available
test=develop

* Fix api.spec
test=develop

* fix api doc
test=develop

test=develop

Fix dygraph show style for FluidDoc.

* add dependecy of collective_helper

* test=develop
fix dependecy of collective_helper