* add DenseNet

* [hapi] support dygrapg amp O2

* fix problem of static pure fp16 in hapi

* fix bug

* fix format

* fix ut

* follow comments

* update ut

* update amp save/load

* fix ut

* refine code format

* add inceptionv3
Co-authored-by: Ainavo <ainavo@163.com>
Co-authored-by: Npithygit <pyg20200403@163.com>

* fix async_read bug

* change index place to cpu

* add tensor size judge

* add async_read & async_write test

* fix bug in async_write

* fix mac py3 ci

* fix bug for cpu version paddle

* fix windows ci bug

* change input argument error type

* change const_cast to mutable_data

* add async_write out-of-bound check and consumate error hint

* fix a small bug for dst_tensor

* add docs and refine codes

* refine docs

* notest,test=windows_ci

* fix windows ci

* fix require

* fix code-block

* add core.is_compiled_with_cuda()

* add resnext model
* add zh docs
* add unittest
* test performance
Co-authored-by: Ainavo <ainavo@163.com>
Co-authored-by: Npithygit <pyg20200403@163.com>
Co-authored-by: Ainavo <ainavo@163.com>
Co-authored-by: Npithygit <pyg20200403@163.com>

* add alexnet

* add roi_align in vision/ops.py

* add roi pool

* rename input as x

* add dlpack api and fix a from_dlpack 

* fix paddle.summary's bug when output contains non-tensor

* add input option in model.summary

This reverts commit 6c110344.

* add num_iters in fit/evalate, test=develop

This reverts commit b2afc8df.

* update 2.0 public api in vision

* fix some flake8 errors

* fix Undefined variables

* fix Undefined variables

* add wget option in download

* fix retry in download when exception occurs

* add to_grayscale, normalize

* add rotate

* add vfip and hflip

* add crop center_crop


* add padding, support constant, reflect, replicate, circular same as paddle.pad

* add get-image-[n,c,w,h] axis utils

add __all__=[] to python files not in API public list; import * only support in API public list files (#32643)

* add op read_file and decode_jpeg

* Modified params of some APIs to support tuple and list.

* fixed bug.

* add Hub Module for easy to use pre-trained models.
*   support list, load, help fucntions.
*   support load models by github, gitee, local 
Co-authored-by: NLielinJiang <jianglielin@baidu.com>

* make hapi support amp, and add unittest

* make unittest only support GPU

* update parameters for amp in hapi.Model

* update hapi.Model.prepare interface, and update unittest

* fix test_model.py unittest bug

* add grad clear in dygraph

* use_fp16_guard defaults to True, which could avoid nan

* add input check, and add internal doc link to low level api

* update doc, and decrease the sample num of dataset to avoid timeout

* make hapi amp param  support str 'O1' or 'O2'

* resume calling , modify the code of the check part

* upgrade the usage of Fleet API, and disable 'pure_fp16' param

* fix warning and no grad