* Support to use external function test=develop

* refine the parms of ast_to_func test=develop

* support to modify dict and list in while_loop test=develop

* polish code test=develop

test=develop

* fix div zero test=develop

* fix div zero test=develop

* add hostdevice function test=develop

* add eps when is zero test=develop

1. Add support for Break and Continue in Dygraph to Static
2. Also add support for gast.Not in NodeTestTransformer
3. Also add support for logical op transformation in LoopTransformer

initialize global nccl context in dygraph
test=develop

* don't add unregisterd attr, test=develop

* add some unittests,test=develop

This reverts commit 66fce9e8.

* remove unnecessary prepare data, test=develop

* Op in while block will not skip PrepareData, test=develop

* Support and/or in controlFlow if test=develop

add approximation for gelu, default value is False (only kernel with eigen is added, remove code for computing gelu with MKLDNN temporarily)

test=develop

* test=cpu

* test=cpu

* test=py2gcc8

* test=py2gcc8

* test=py2gcc8

* test=py2gcc8

* test=py2gcc8

* test=py2gcc8

* test=py2gcc8

* test=develop;test=document_fix;test=py2gcc8

Fleet Parameter Server API Integrated

* Improved stability through the use of caching

test=develop

* fix style

test=develop

* Fix syntax error

test=develop

* Fix syntax error

test=develop

* Support Tensor.shape in control flow if/for/while and separate TensorShapeTransformer from BasicApiTransformer. test=develop

* remove the fill_constant_batch_size_like,  gaussian_random_batch_size_like and uniform_random_batch_size_like_cn three Op unittest 

* add transpose_flatten_concat_fuse_pass tests for gpu and trt, test=develop

* update test_inference_api.py, test=develop

* Support and/or in controlFlow if test=develop

* Refine IsControlFlow interface test=develop

* fix the travic ci problem

* add fusion group test for backward and refine code

* Support that the input(x) of stack api is a LoDTensorArray. test=develop

* Support that the input of concat api is a LoDTensorArray. test=develop

* Add tests to test stack/concat called in dygraph_to_static. test=develop

* fix bug which input shape, test=develop

* add error type,test=develop

* add dygraph parallel env doc, test=develop

* polish details, test=develop, test=document_fix

* fix examples error in other apis, test=develop

* fix more example error in other api, test=develop

* add white list for gpu examples, test=develop, test=document_fix