* native commit for triple grad of sigmod

* Updated unittests files

* init functional jacobian api

* Updated trible_test func

* Updated gradient_checker & test_script

* finish test with dtype float32

* add float64 test case

* polish code

* use atol=1e-5 with dtype float64

* fix for ci

* set timeout for test_jacobian

* fix dygraph grad to support high differential

* polish API docstring

* Updated gradient checker and some related files

* fix double grad strip error for high differential

* fix double grad strip error for high differential

* Add Sigmoid triple grad tests

* fix dygraph double grad dtype error when calling for high differential senario

* Updated triple grad teses func

* Use np.random to initialize ddx

* Updated triple_grad_check func

* add todo for gradient checker and refine some comments

* remove additional code

* add test for warnging in backward.py

* format python code
Co-authored-by: Nveyron95 <veyron_wu@163.com>
Co-authored-by: Nlevi131 <limaolin01@baidu.com>

* fix the travic ci problem

* don't expose numerous Tensor.set(), test=develop

* fix condition, test=develop

* fix float16 bug, test=develop

* feed should be Tensor or np.array, not Variable or number, test=develop

* use forcecast to copy numpy slice to new array, test=develop

* remove float16-uint16 hacking, test=develop

* instag lod tensor impl

* First PR for instag

* First PR for instag

* Before adding Selection Rows.

* Change name from instag to filter_instag, add upgrade the impl of filter_instag

* Change name from instag to filter_instag, add upgrade the impl of filter_instag

* Fix yapf error in gradient_checker.py to pass Travis-CI

* Fix Filter Instag Grad test=develop

* Fix Filter Instag Grad test=develop

* 1) Fix API.spec, add filter_instag Op. 2) Add Vector Support for CUDA. test=develop

* Impl Loss_weight and empty output handler

* change Loss Weight datatype to Float32, and add Loss Weight as 2nd output

* 1) Support Tensor Input(without LOD) 2) Add Unit test

* Filter By Instag Final test=develop

* Update API.spec for filter_by_instag test=develop

* Update API.spec for filter_by_instag 2 test=develop

* Add Filter By Instag Coverage

* code format of test_layers.py

* code format test_layers.py test=develop

* Make API args more readable test=develop

* Make API args more readable and pass code format test=develop

* Filter By Instag Op, Rename Map to Index Map test=develop

* Filter By Instag Op, code format err in filter_by_instag_op.cc  test=develop

* Filter by instag op: code format of cpp files test=develop

* Filter by instag Op: Api spec modification test=develop

* Filter by instag Op: Api spec doc id modification test=develop

* Filter by instag Op: Api spec and doc preview  test=develop test=document_preview

* Filter By Instag Op, fix doc erro test=document_preview test=develop

* Filter By Instag Op, fix doc err and Api spec test=document_preview test=develop

* Filter By Instag Op, fix Api spec test=document_preview test=develop

* Filter By Instag Op, fix Paddle Encoforce deprecated warning test=document_preview test=develop

* Filter By Instag Op, fix Paddle Encoforce deprecated and code format warning test=document_preview test=develop

* Update backward.py:
     - If there is no input grad var in all outputs of previous ops, do not append this op into graph.
     - Only apply this stragety when double backward.
* Update some double backward op.
* Update sum_op to judge whether a tensor is empty by numel or IsInitialized().

* add elementwise_add_grad_grad op. test=develop

* use defined GradMaker. test=develop

* Add conv2d_grad_grad_op
* Extracte the cuDNN conv algo searching code in conv_cudnn_helper.h.
    - Now use it in conv2d_grad_grad.
    - Will simply the searching code in conv2d and conv2d_grad in next PR.
* Enhance and fix bug in unit testing of gradient_checker.
* Support to fetch empty variables，return None in Python.

Support backward of backward for Relu and add a new gradient checker by comparing theoretical and numerical Jacobian. (#16862)

* Support backward of backward and a new gradient checker
* Rename decorators.py to decorator_helper.py, since Python on Windows CI has decorators package.

1. Add ReluDoubleGradMaker when register relu_grad.
2. Add a new gradient checker by comparing theoretical and numerical Jacobian.  Check double gradients by double_grad_check.