add faster rcnn prune config (#3599)

configs/slim/prune/faster_rcnn_r50_fpn_prune_fpgm.yml

+pretrain_weights: https://paddledet.bj.bcebos.com/models/faster_rcnn_r50_fpn_1x_coco.pdparams
+slim: Pruner
+
+Pruner:
+  criterion: fpgm
+  pruned_params: ['conv2d_27.w_0', 'conv2d_28.w_0', 'conv2d_29.w_0',
+                  'conv2d_30.w_0', 'conv2d_31.w_0', 'conv2d_32.w_0',
+                  'conv2d_33.w_0', 'conv2d_34.w_0', 'conv2d_35.w_0',
+                  'conv2d_36.w_0', 'conv2d_37.w_0', 'conv2d_38.w_0',
+                  'conv2d_39.w_0', 'conv2d_40.w_0', 'conv2d_41.w_0',
+                  'conv2d_42.w_0', 'conv2d_43.w_0', 'conv2d_44.w_0',
+                  'conv2d_45.w_0', 'conv2d_46.w_0', 'conv2d_47.w_0',
+                  'conv2d_48.w_0', 'conv2d_49.w_0', 'conv2d_50.w_0',
+                  'conv2d_51.w_0', 'conv2d_52.w_0']
+  pruned_ratios: [0.1,0.2,0.2,0.2,0.2,0.1,0.2,0.3,0.3,0.3,0.2,0.1,0.3,0.4,0.4,0.4,0.4,0.3,0.4,0.4,0.4,0.4,0.4,0.4,0.4,0.4]
+  print_params: False