* cherry-pick update document

* fix all documentation

* fix docs conf.py

* fix some error

* fix readthedocs

* split yolov3 loss

* Polish Reader to simplify preprocessing logic.
#  sample_transforms-> make batch -> batch_transforms in Reader.
* Clean some code
* Imporve yolov3_r50vd_dcn_obj365_pretrained_coco 41.4 to 41.8.
* Update all configs.

* add export_model doc

* refine doc for transfer learning

* Error out if fp16 is enabled and affine channel is used

* Add more documentation on mixed precision training

* Document mixed precision training flags

* Clarify GPU is required for mixed precision training

* 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

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.

* update doc in PaddleDetection

* add save best model & fix dcn configs

* fix cn docs

* fix easy problem

* update GETTING_STARTED.md

Add PaddleDetection docs & some model configs

* fix infer doc and config default

* refine dataset exists

* fix None proc

* add cn doc for INSTALL/GETTING_START

* cn for GETTING_STARTED

* fix doc