* upgrade code to 2.0-beta

* add high level api doc

* add define callback/metric/loss chapter

* add define callback/metric/loss chapter

* add save_model

* update api

* change api name

* change api name

* fix_some_wrongs

* fix_logs

* change from Pool2D to AdaptiveAvgPool2d in image_search

* hello paddle updated

* convnet_image_classification updated

* dynamic_graph updated

* convnet_image_classification updated

* imdb bow classification updated

* fix_test_loader

* fix_mnist_wrongs

* upgrade code to 2.0-beta

* add high level api doc

* seq2seq with attention added

* image_search use latest API

* update seq2seq with attention

* image segmentation with u-net notebook

* update segmentation notebook

* upgrade code from 2.0-alpha to 2.0-beta

* add getting started doc

* delete invalid files

* image segmentation with u-net notebook

* update segmentation notebook

* upgrade code from 2.0-alpha to 2.0-beta

* image segmentation with u-net notebook

* update segmentation notebook

initialize paddle2.0_docs directory