* Support finetune by custom dataset

* add finetune args

* add load finetune

* reconstruct load

* add transfer learning doc

* add fruit demo

* add quick start

* add data preprocessing FAQ

* Disable optimizer fusion for RCNN models

waiting for upstream fix

* Disable completely

* Add support for mixed precision training

* Disable `fuse_all_reduce_ops` when training in mixed precision

waiting for upstream fix

* Initial support for distributed training

* Housekeep on rank 0 only

* Conform to models convention

* fix rcnn eval on voc

* update comment

* refine auto download logs

In PaddleDetection,  making use of tb-paddle to record data, so as to display in TensorBoard.  (#3295)

* use tb_paddle to log data
including:

* loss
* mAP
* image

* modified:   requirements.txt
* modify the missing argument --use_tb
* modify GETTING_STARTED.md and docs/GETTING_STARTED_cn.md

add the argument of tb-paddle in docs.
* optimize the display name of image displaying.

* Add benchmark (fps) in MODEL_ZOO doc

* fix add map_type

* minor fix for PaddleDetection

* fix bug in sharedmemory
* fix bug in sharedmemory

* add cascade gn

* add cascade mask gn config

* update configs

* fix train on VOC on multi-GPU hang

* not set persistable in parse_fetches

* fix rpn_only in cascade mask fpn

* fix rpn_only in cascade rcnn

* add gn doc

* refine cascade models

* update model zoo doc

* cascade-mask-fpn add

* add a global norm_type flag

* add group norm on fpn

* add gn on box head

* add faster rcnn fpn 50 gn config

* add mask branch norm

* update configs

* add yolov3_darknet cinfig comment

* fix commment

* add log_iter

* Add full configs example & refine doc

* refine docs

* add_ssd_config_demo