* Implementation of WandbLogger callback to log metrics and model checkpoints

* Added documentation on how to use WandbLogger and VisualDL

* Minor bug fix and pull from upstream

* Removed pass statement for cleanup

* Update logging_en.md

Fixed documentation

* Update trainer.py

* polish args

* fix import bugs

Besides removing unnecessary imports, this also fixes the missing logger module in test_architectures.py within the static directory.

* fix sync_bn in cpu

* fix some docs error

* fix warning

* add Distill model
* add distill+prune

* unity weights

* add VisualDL and save best_model

* add VisualDLWriter callback

* add blazeface & update rcnn modelzoo

* remove condition num_priors

* add fp16 and fleet api

* add instructions for fleet api and fp16

* modify example of fleet api

* modify docs of fleet api

* eval in train

* add prune dygraph, test=dygraph

* fix slim readme and prune

* ppdet installable

Merge dygraph branch into master branch.
Co-authored-by: NFDInSky <48318485+FDInSky@users.noreply.github.com>
Co-authored-by: Nsunxl1988 <47514455+sunxl1988@users.noreply.github.com>
Co-authored-by: Nwangguanzhong <jerrywgz@126.com>
Co-authored-by: Nwangxinxin08 <69842442+wangxinxin08@users.noreply.github.com>
Co-authored-by: Nwanghuancoder <wanghuancoder@163.com>
Co-authored-by: Ncnn <liuhui29@baidu.com>
Co-authored-by: NGuanghua Yu <742925032@qq.com>
Co-authored-by: NKaipeng Deng <dengkaipeng@baidu.com>
Co-authored-by: Nroot <root@bjyz-sys-gpu-kongming0.bjyz.baidu.com>