[doc] fix doc about FGD distill train (#6326)

* add FGD distill code

* add configs

* add doc

* fix pretrain

* pre-commit

* fix ci

* fix readme

* fix readme

* fix ci

* fix param groups

* fix

* fix doc

* fix doc,test=document_fix

configs/retinanet/README.md

@@ -6,12 +6,13 @@
 | ------------ | --------- | -------- | ----------- | --- | ------ | ---------- | ----------- |
 | ResNet50-FPN | RetinaNet | 2        | 1x          | --- | 37.5   | [model](https://bj.bcebos.com/v1/paddledet/models/retinanet_r50_fpn_1x_coco.pdparams) | [config](./retinanet_r50_fpn_1x_coco.yml) |
 | ResNet101-FPN| RetinaNet | 2        | 2x          | --- | 40.6   | [model](https://paddledet.bj.bcebos.com/models/retinanet_r101_fpn_2x_coco.pdparams) | [config](./retinanet_r101_fpn_2x_coco.yml)  |
-| ResNet50-FPN | RetinaNet | 2        | 2x          | --- | 40.8    | [model](https://bj.bcebos.com/v1/paddledet/models/retinanet_r101_distill_r50_2x_coco.pdparams) | [config](./retinanet_r50_fpn_2x_coco.yml)/[slim_config](../slim/distill/retinanet_resnet101_coco_distill.yml) |
+| ResNet50-FPN | RetinaNet + [FGD](../slim/distill/README.md) | 2        | 2x          | --- | 40.8    | [model](https://bj.bcebos.com/v1/paddledet/models/retinanet_r101_distill_r50_2x_coco.pdparams) | [config](./retinanet_r50_fpn_2x_coco.yml)/[slim_config](../slim/distill/retinanet_resnet101_coco_distill.yml) |
 
 
 **Notes:**
 
-- All above models are trained on COCO train2017 with 8 GPUs and evaludated on val2017. Box AP=`mAP(IoU=0.5:0.95)`.
+- The ResNet50-FPN are trained on COCO train2017 with 8 GPUs. Both ResNet101-FPN and ResNet50-FPN with [FGD](../slim/distill/README.md) are trained on COCO train2017 with 4 GPUs.
+- All above models are evaludated on val2017. Box AP=`mAP(IoU=0.5:0.95)`.
 
 
 ## Citation