[Fix] Fix WIDERFace SSD loss NAN. (#9734)

Author: RangiLyu

configs/wider_face/ssd300_wider_face.py

@@ -5,7 +5,7 @@
 model = dict(bbox_head=dict(num_classes=1))
 # optimizer
 optimizer = dict(type='SGD', lr=0.012, momentum=0.9, weight_decay=5e-4)
-optimizer_config = dict()
+optimizer_config = dict(grad_clip=dict(max_norm=35, norm_type=2))
 # learning policy
 lr_config = dict(
     policy='step',