* add ppyolov2

* fix bugs and modify docs

* modify code and doc according to review

* fix bugs while resolving conflicts

* add PP-YOLO tiny

* add pssdet model

* fix name of pretrain weights

* format, test=document_fix

* add senet

* add annotations according to review

* Add S2ANet model for Oriented Object Detection.

* update slim model

* fix skip quant

* add docs for custom dataset, reader and model technical

* modify docs according to review, test=document_fix

* modify READER.md, test=document_fix

* fix warning

* refine ppdet file name

* unify bbox_utils

* add Distill model
* add distill+prune

* update files

* update for ci-travis

* add fcos ttfnet coment, update modelzoo

* fix ttfnet coment

fis the issue in function _get_shared_memory_size_in_M in the case of that there are multiple 'shm' space (#2434)

* update DataLoader & speed up Mask RCNN

* refine trainer

* support not pad gt in rcnn model

* add comment in reader