* add astype api in Variable and refine code style

* add astype api in Variable and refine code style

* add astype api in Variable and refine code style

* refine doc

* polish cn document of unique_name

* polish guard and switch

* add example code

* add filter to simplify optimizer api doc

* update related optimizer doc

* refine prepare steps doc, test=develop

* replace layers.data with fluid.data, test=develop

* update softmax_cn.rst, test=develop

* update softmax_cn.rst, test=develop

* fix error, test=develop

* add comment for name, test=develop

* update softmax_cn.rst, test=develop

* update softmax_cn.rst, test=develop

* fix softmax_cn.rst, test=develop

* fix softmax_cn.rst, test=develop

* polish model average zh doc api

* polish details

* polish based review commment

* fix indent

* fix the doc format of io.chain.

test=develop
test=document_preview

* fix fluid.data code example, test=develop, test=document_preview

* skip PipeReader in IO

* skip PipeReader in IO
test=develop

* fixed bugs about PipeReader

change fluid.layers.data to fluid.data in api_cn for retinanet_target_assign, sigmoid_focal_loss, retinanet_detection_output. (#1473)

* test=document_preview
change  to  in api_cn for retinanet_target_assign, sigmoid_focal_loss, retinanet_detection_output.

* test=document_preview
remove Notice: this OP supports CPU mode only in api_cn for retinanet_target_assign and retinanet_detection_output

* fix DataFeedDesc set_batch_size, set_dense_slots cn api doc test=document_preview

* fix continuous_value_model op cn doc

* fix some rst error

* add cpu support only in cvm op

* add explain of N in cvm

* refine lstm

* refine LSTMs, test=develop

* fix notice color, test=develop

fix the API doc of paddle.fluid.layers.array_length

* fix the document of API:fluid.Layers.array_length

* fix the document of API:fluid.Layers.array_length

fix the document of API:fluid.Layers.array_read

* fix the document of API:fluid.Layers.array_read

* fix the document of API:fluid.Layers.array_read

fix the API doc of paddle.fluid.layers.array_write

* fix the document of API:fluid.Layers.array_write

* fix the document of API:fluid.Layers.array_write

* fix the document of API:fluid.Layers.array_write

* fix the document of API:fluid.Layers.array_write

* doc fix

* update doc/fluid/api_cn/layers_cn/add_position_encoding_cn.rst
update doc/fluid/api_cn/layers_cn/beam_search_cn.rst
update doc/fluid/api_cn/layers_cn/beam_search_decode_cn.rst
update doc/fluid/api_cn/layers_cn/chunk_eval_cn.rst
update doc/fluid/api_cn/layers_cn/dynamic_gru_cn.rst
update doc/fluid/api_cn/layers_cn/gru_unit_cn.rst
update doc/fluid/api_cn/layers_cn/lstm_unit_cn.rst
update doc/fluid/api_cn/nets_cn/scaled_dot_product_attention_cn.rst

* Update format for dynamic_gru_cn, gru_unit_cn and scaled_dot_product_attention_cn.

* build strategy api doc

* fix transpiler api doc: get_trainer_program / get_pserver_program / get_pserver_programs

* delete useless word

* add default value of wait_port

fix doc according to reviews.test=develop

This reverts commit a29dcbec.

* fix io_batch develop=test

* fix io_batch develop=test

* rewrite batch_cn.rst test=develop

* rewrite batch_cn.rst test=develop

* add instance norm

* Fix problems of chinese docs

* Revise chinese docs

* Add more fixes

* Add data type comments for some inputs

* fix PyReader develop=test

* rewrite PyReader_cn.rst develop=test

* dist training on gpu, test=document_preview

* fix the doc

* fix doc

* fix doc

* add the link

* test=develop test=document_preview