add r18

configs/pascal_voc/cascade_rcnn_r18_fpn_1x_voc0712.py

+_base_ = './cascade_rcnn_r50_fpn_1x_voc0712.py'
+model = dict(
+    backbone=dict(
+        depth=18,
+        init_cfg=dict(type='Pretrained',
+                      checkpoint='torchvision://resnet18')),
+    neck=dict(
+        in_channels=[64, 128, 256, 512],
+        ))