test=develop

This reverts commit 6f6cc947.

* modify doc for Attr(data_format) or Attr(data_layout) and add Raises information, test=develop

* modify doc of conv3d_transpose for Raises information, test=develop

* add recompute document

* fix dead links for models

* fix_dead_links

update x2paddle link

* update DetectionMAP reference

* Update DetectionMAP_cn.rst

* fix-apis-develop

* fix-one-error

* remove Fake and fix layers_cn.rst

* fix-apis-develop

* fix-one-error

Add the description and example of shape tensor supporting int32

fix-contents_cn

* add dataset

* add back dataset and data feeder

* fix Layer and Pool2D api, test=develop

* update label_smooth_cn.rst, test=develop

* Polish cn doc fo gru_unit, lstm_unit and scaled_dot_product_attention.

* Add gather_tree_cn.rst and update the data type of assign/fill_constant_batch_size_like/gather_nd.

* Fix _cn_api_fluid_layers_gather_tree

* Update tensor_array_to_tensor and add mse_loss.

test=document_fix

* fix Layer and Pool2D api, test=develop

* update label_smooth_cn.rst, test=develop

* add save load save_dygraph load_dygraph, test=develop

* add .idea in gitignore, test=develop

* minor fix, test=develop

* doc refines, test=develop

* doc refines, test=develop

* Update save_load_variables.rst (#1520)

* update 1_6 release

* fix readme
test=document_preview

* add new tree index

* update 1_6 release

* fix readme
test=document_preview

* add new tree index

* add new seq2seq api_cn, test=document_preview

* add seq2seq api_cn, test=document_preview

* add seq2seq api_cn, test=document_preview

modify fluid.layers.data to fluid.data and Synchronize Chinese doc and English doc

Fix fluid.layers.data of example code, Synchronize Chinese and English DOC of API:layers.sum.

* Refine the Chinese documentation of DynamicRNN.
test=develop
test=document_preview

* Refine the memory interface of DynamicRNN.
test=develop
test=document_preview

* Refine update_memory and output of DynamicRNN.
test=develop
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* Update the format of note and fix a error in example.
test=document_preview

* Add name and correct some examples.

* Refine the documentation according to the review comments.

* Add function list and the doc of function __call__.

* Add optional for x in step_input.
test=develop

* Update sub-titles.

* Change fluid.layers.data in examples to fluid.data.

* Add examples for step and static input.
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* Refine some sentences.
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* Add description of shape and data type for input and output variables.
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