* prun backward ops
test=develop

* add not_been_used_vars to no_grad_set
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().

Fix the following API examples:

paddle.fluid.scope_guard
paddle.fluid.backward.append_backward
paddle.fluid.cpu_places
paddle.fluid.cuda_pinned_places
paddle.fluid.cuda_places
paddle.fluid.in_dygraph_mode
paddle.fluid.CUDAPlace
paddle.fluid.CPUPlace
paddle.fluid.CUDAPinnedPlace

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.

* fix some grad op desc maker
test=develop

* fix grad op desc makers
test=develop

test=develop

test=develop

* hide operator API

* update

* update api.spec

* fix merge

* fix test

* hide program APIs

* fix merge error

* update

Fix pb_protobuf2 FromString problem

Fix str and bytes problem
Fix sorted problem
Fix math problem
Fix CI problem

Revert part of a3ca4c99

* remove block api

* remove clone_variable

* hide block inner apis

* update

* fix tests

This reverts commit 4d8e8ee2, reversing
changes made to d6a9f005.