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As the title

This PR provides very basic and simple framework for transforming Dygraph to Static Graph.

API names, final outputs are not determined yet. Feel free to modify or add class/function/type when you think the framework is not extendable for you.

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* fix circular dependent

* try import layers.nn from dygraph
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* allow sublayer or param shadow attrs

* add unittest
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* change remove fn name
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* DataParallel state dict don't include _layers.; test=develop

* add unitest of data parallel; test=develop

* add load state test; test=develop

* fix save_dygraph with nonexistent dir
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* minor fix
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* fix unittest
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* fix static save
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* use snake_cased Layer name

* add named_parameters and named_sublayers api

* add include_sublayers param, add unittest
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* fix named unittests
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* fix unittest
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* add api docs
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* arg fix
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* reserve rnn_impl name_scope for static graph
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* fix load static param
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* fix load static param
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* add clear_gradients for Optimizer, add api description, test=develop

* fix optest for optimizer's clear_gradient interface, test=develop

* add sample code, test=develop

* polish sample code, test=develop

* update layers, test=develop

* update layers for resnet, test=develop

* fix is_test attr, test=develop

* update cycle_gan, test=develop

* update reinforcement_learning, test=develop

* update ocr, test=develop

* fix bug, test=develop

* update PRelu arg
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* fix unittests
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* fix element mode alpha shape
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* split channel_or_input_shape arg
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* fix unittest
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* modify fc to linear in sample code, test=develop

* remove FC, test=develop

* remove warnings, test=develop

* drop fluid/imperative/README.md , test=develop

* change fc to linear, test=develop

* polish code style, test=develop

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* support dygraph structured name; test=develop

* add load static param unitest and fix save load; test=develop

* fix varBase import error; test=develop

* fix unitest error; test=develop

* add comment for parameter; test=develop

* fix uni test error; test=develop

* change parallel se-resnet; test=develop

* fix dygraph se resnext parallel test; test=develop

* remove useless code; test=develop

* remove useless code; test=develop

* add dygraph LayerList

* add unittest
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* add newline
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* typo fix
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* move private weight properties to public ones
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* revert changes to FC
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* fix unittest
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* fix unittest
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* fix coverage
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* fix merged dev
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* bug fix
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* expost fluid.dygraph.TracedLayer apis, test=develop

* polish doc, test=develop

* follow comments, test=develop, test=document_fix

* follow comments, test=develop

* remove save_inference_model return value, test=develop

* update layers used in mnist dygraph model, test=develop

* fix import issue, test=develop

* add dygraph utils, test=develop

* add unittest, test=develop

* remove params in Tracer object, test=develop

* Repair failed optest, test=develop

* remove build_once & name_scope (Conv2D)
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* fix unittest
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* Conv2DTranspose

* Conv3D & Conv3DTranspose
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* Pool2D & BatchNorm

* Embedding

* LayerNorm

* GRUUnit & NCE

* PRelu

* BilinearTensorProduct

* GroupNorm & SpectralNorm

* TreeConv
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* fix LayerNorm in transformer unnittest
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* disable LayerNorm or BatchNorm in multicard
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* refine Layer.create_parameter api
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* refine LayerNorm, remove begin_norm_axis param, add normed shape check
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* LayerNorm bug fix
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* fix optest,test=develop

* fix optest, test=develop

* fix optest for pass parameter_list when constructing an Optimizer class instance, test=develop

* polish code for better code style, test=develop

* fix se_resnext optest, test=develop

* polish code for better code style, test=develop
Co-authored-by: Nsongyouwei <youwei0314@gmail.com>

* add ParameterList container

* add unittest
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* fix container, rebase remove build_once
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* add parameters None check
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* update Sequential unittest
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* use Linear in Sequential sample code
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* fix Sequential arg type doc
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* update layers, test=develop

* fix input numpy, test=develop

* fix bugs, test=develop

* follow commments, test=develop

* update getitem, test=develop

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* add Linear layer
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* update unittest for coverage
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adds unary operator __neg__ for VarBase in dygraph mode, and for Variable in static graph mode.

* remove build_once & name_scope (Conv2D)
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* fix unittest
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* Conv2DTranspose

* Conv3D & Conv3DTranspose
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* Pool2D & BatchNorm

* Embedding

* LayerNorm

* GRUUnit & NCE

* PRelu

* BilinearTensorProduct

* GroupNorm & SpectralNorm

* TreeConv
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* fix LayerNorm in transformer unnittest
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* disable LayerNorm or BatchNorm in multicard
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* refine Layer.create_parameter api
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* refine LayerNorm, remove begin_norm_axis param, add normed shape check
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* LayerNorm bug fix
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* patch math method for varbase using auto-generated op functions, test=develop

* clean code that handles batch_size, test=develop

* follow comments, test=develop

* follow comments, test=develop

* code clean, test=develop

* refine init function, test=develop

* add tests, test=develop

* remove extern, which may cause symbol error in gcc-4.8, test=develop

* 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

* dygraph Embedding layer use lookuptable v2
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* fix test_nce
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* add param & grad shape check for sgd op

* add _reshape_inplece interface for dygraph parallel

* refine unittest based paddle/models scripts, test=develop

* add unittest for parallel grad fuse, test=develop

* add numpy bridge

* fix template compile

* add unittest, add default
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* fix unittest
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* fix unittest
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* zero_copy=True for to_variable,
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* bug fix
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* disable deprecated NumPy API
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* use better design of NumpyAllocator
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* fix Py_None check
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* reset c++ tracer when jump out dygraph guard
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* refine PADDLE_ENFORCE_xx format
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* bug fix of tracer switch
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* update decref
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* polish jit trace codes, test=develop

* polish codes again by removing var_id, test=develop

* friendly trace interface, test=develop

* refine TracedLayer, test=develop

* add some docs, test=develop