Add basic while_loop

Add tests to use dy/dx to make sure the gradient values calculated by the control flow backward is correct. Also fixed bugs detected by those tests.

Fix bugs:

1. Unlike sum_op, optimizer ops don't allow uninitialized input tensor. But in conditional_block_grad_op, since the conditional_block may not run, the output gradient tensor may be uninitialized, which will cause the optimizer op error. To fix it, we should let optimizer ops support uninitialized input like sum_op or assign the uninitialized gradient to 0 when the conditional_block_grad_op doesn't run. I found there are about 10+ optimizer ops. **To be simpler, I just assign output gradient of the conditional_block_grad_op to 0 in this PR**. But it can be further explored whether we can make optimizer ops like sum_op to support uninitialized input tensor because theoretically we can speed up without the assigning in conditional_block_grad_op.

2. Infer parameter shapes during append_backward. I didn't know that all our parameters are in global block. When op_desc is inferring shapes at the sub-block, it may not know the shape of gradients of parameters whose shape information is at global block. I fixed it by inferring shapes of gradients from forward var.

This PR also did some code clean up:
1. Print the var name when sgd_op catches shape error so that it is easier to debug
2. Fix a typo: dicta -> dict

    Add a python interface for Gelu.
    Add documentation for fluid.layers.gelu.

* dygraph mode support linear lr warm up; test=develop

* add unitest for linear warmup; test=develop

* add input type check; test=develop

* fix type check assert error; test=develop

* change type error; test=develop

* test=develop, fix docker with paddle nccl 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

* add variable method to varbase and refactor to_variable to support return varbase

* support kwargs in varbase constructor

* add VarBase constructor to support default python args

* refine varbase initial method

* reset branch

* fix ut for change VarBase error info to PaddleEnforce

* cherry is parameter change before

* overload isinstance to replace too many change of is_variable

* rm useless files

* rm useless code merged by git

* test=develop, fix some ut failed error

* test=develop, fix test_graph_wrapper

* add some tests, test=develop

* refine __getitem__, test=develop

* add tests, test=develop

* fix err_msg, test=develop

* fix fill_constant_batch_size_like_op precious problem  test=develop

* set dim[0] to -1 if dim[0] < 0 and remove assertion to runtime, test=develop

* modify ENFORCE message, test=develop

* add validation for x.shape[0] > 0, test=develop

* add ut, test=develop

* fix adam sample code bug test=document_fix

* fix sample code bug in scale test=document_fix

This reverts commit 007c9975.

* refactor AUC OP and add its CUDA Kernel
* the layout of global auc doesn't change

* fix the device supported of the op unique and unique_with_counts.
test=develop
test=document_fix

* Fix the precision of test in the op of unique and unique_with_counts.
test=develop
test=document_fix

Add English doc for cond

* Commit before merging develop

test=develop

* Backup after working with Huihuang logs

* Commit before deleting Huihuang debug loggings

* Commit before debug

test=develop

* Fix bug commit

test=develop

* Backup of fixing bugs

test=develop

* Clean up code

test=develop

* Fix a bug in sum_op

test=develop

* Add ascending for argsort

* Refine api doc description.

* Refine descending description

* Add int32 logic to speedup when data is small size.

* Remove int32 opt as not support in python

* add Adam beta1/beta2 support Variable. test=develop

* batch_norm momentum support variable. test=develop

* fix format. test=develop

* add batch_norm momentum variable example. test=develop

* move MomentumTensor to training branch. test=develop

* split example. test=develop

* fix doc. test=develop

* fix PADDLE_ENFORCE ci. test=develop

* fix format. test=develop

* support data_norm_op run in CUDA
* add two parameters sync_stats & summary_decay_rate
* add UT

* add the framework support for distfc and ut, test=develop
* fix the implementation of shard_index_op, test=develop

* fix the fill_constant op precious problem test=develop

pass the argument `allow_out_of_range` of one_hot op to c++ back end.

* add input type and input data type check for Print_op test=develop

* add control flow API: case. test=develop

* delete 'raise TypeError' in _error_message() and return a string. test=develop

* polish API document. test=develop

* fix auc drop first commit test=develop

* update datanorm op

* update datanorm with enforce test=develop

* update test=develop

* update format test=develop

* update format

* update format test=develop

* add unit test test=develop

* update unit test test=develop

* update format test=develop

* update format test=develop

* update API description test=develop

* update API description test=develop

* update format test=develop

* fix codes as comments test=develop

* fix description as comments test=develop

* fix description as comments test=develop

* update codes.. test=develop

* modified error message for conv and conv_transpose, test=develop

* modified doc of conv and conv_transpose op, test=develop

* modified the expression for error message, test=develop

* modified error message for group_norm op, test=develop

* modified detail of Attr(data_format) or Attr(data_layout)

* add ValueError in API doc for maxout op, test=develop

* add API switch_case. test=develop

add Nest

* modify code according to reviews:
1.Attr(branch_index) support 'uint8' and 'int64' besides 'int32'.
2.remove useless code.
test=develop

* replace fluid.layers.data with fluid.data and polish API document. test=develop

* add input type and dtype check template, and update some APIs check

* refine check template, and update some APIs check in nn.py

* update some APIs check in loss.py

test=develop

* Split some APIs from nn.py to loss.py

test=develop

* fix test_detection unit-test

test=develop