Fix rpn_target_assign API.

Author: jerrywgz

python/paddle/fluid/layers/detection.py

@@ -164,7 +164,7 @@ class number, M is number of bounding boxes. For each category
         })
 
     # 4. Reshape and gather the target entry
-    scores = nn.reshape(x=scores, shape=(-1, 1))
+    scores = nn.reshape(x=scores, shape=(-1, 2))
     loc = nn.reshape(x=loc, shape=(-1, 4))
     target_label = nn.reshape(x=target_label, shape=(-1, 1))
     target_bbox = nn.reshape(x=target_bbox, shape=(-1, 4))