We found that the reason of multiple return error in SimNet is that I wrote wrong task_mode. If we set task_mode as "pairwise" correctly, which is a format of the model input data, the multiple return won't have problem in the unit test. In this PR we corrected the task_mode and enable multiple return in SimNet unit test.

refactor fleet api under paddle.fleet
update DistributedStrategy

* add reinforcement learning model test=develop

* align backward test=develop

* add gym in paddle_build.sh test=develop

* rm pip install in script test=develop

* refine paddle_build.sh test=develop

* fix sed error in macOS test=develop

* polish code test=develop

* refine infer ir pass tests, test=develop

* commit to change copy_from_cpu input to np array, test=develop

Enable multiple "return" statement in MNIST as Unit test in dy2stat.

* fix test_activation_op, test=develop

* remove paddle.mul related unittest, test=develop

* delete mul op. test=develop,test=document_fix

* bug fix. test=develop,test=document_fix

Add Support for SelectedRows for Transpose OP and Fix a Bug That SelectedRows Cannot be Supported in SimNet (#25536)

This PR fixes a bug that SelectedRows cannot be supported in SimNet. The reason of this bug is that dygraph basic_engine didn't copy var's type when the var needs to be accumulated during backward. So when a var is SelectedRows and needs to be accumulated, like SimNet which calls net for two times, the var's type will be changed to default LoDTensor thus bug happens. To fix it, we just also copy the type.

Without this PR, the accumulated SelectedRows parameters in dygraph will be changed into LoDTensor. So when we fixed the bug of supporting SelectedRows in SimNet, we found `test_imperative_lod_tensor_to_selected_rows` failed and threw the error that SelectedRows was not supported for Transpose OP. To fix it, too, this PR also added support for SelectedRows for Transpose OP.

* remove op call stack test=develop

* fix typo test=develop

* fix doc of logical_* op.

* fix doc of op pow.

* fix comment syntax error9D

* fix operator reciprocal demo.

* fix logical_* ops' doc. test=develop,test=document_fix

* bug fix. test=develop,test=document_fix

* bug fix. test=develop,test=document_fix

* bug fix. test=develop,test=document_fix

* bug fix. test=develop,test=document_fix

add try..catch...

* add bilateral slice op

* randint API: remove out, devive, stop_gradient; add name; test=develop

* test=develop

* test=develop

* test=develop

According to paddle 2.0 standard
1, change flip api attr name 'dim' to 'axis'.
2, support empty axis
3, change example code to imperative mode.

According to paddle 2.0 standard.
1, chang api to def meshgrid(*args, **kwargs) we hide name argument in **kwargs.
2, add relate ut.
3, change example code to imperative mode

* experimental refactoring, test=develop

* add TranslatedLayer & remove StaticModelRunner, test=develop

* revert tracedlayer change, test=develop

* fix test_mnist unittest error, test=develop

* add doc & examples, test=develop

* polish doc details, test=develop

* add imperative.jit module, test=develop

* change TranslatedLayer pos, test=develop

* adjust jit module import path, test=develop

* polish doc based review result

* add SaveLoadConfig.separate_params to save paraams separately

* add Layer.buffer support, test=develop

* polish doc details based review result, test=develop

* polish details baesd review comments, test=develop

* add empty str check for param, test=develop

* add unittests, test=develop

* polish details based review comment, test=develop

* remove blanks in comment, test=develop

* polish doc details, test=develop

* update imperative doc link, test=develop

* add api attr for load, test=develop

* make default_collate_fn visible. test=develop

* test_dist_fleet_ctr disable, test=develop

[while grad]Support pruning op in find_op_path about while sub-block when appending backward (#25330)

Prune OPs which are not related with loss in while sub-block when constructing backward OP path.

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