Set stop_gradient=True for some variables in SSD API. (#9396)

python/paddle/fluid/layers/detection.py

@@ -134,6 +134,7 @@ def detection_output(loc,
     scores = nn.softmax(input=scores)
     scores = ops.reshape(x=scores, shape=old_shape)
     scores = nn.transpose(scores, perm=[0, 2, 1])
+    scores.stop_gradient = True
     nmsed_outs = helper.create_tmp_variable(dtype=decoded_box.dtype)
     helper.append_op(
         type="multiclass_nms",
@@ -148,6 +149,7 @@ def detection_output(loc,
             'score_threshold': score_threshold,
             'nms_eta': 1.0
         })
+    nmsed_outs.stop_gradient = True
     return nmsed_outs
 
 
@@ -837,4 +839,6 @@ def multi_box_head(inputs,
         mbox_locs_concat = tensor.concat(mbox_locs, axis=1)
         mbox_confs_concat = tensor.concat(mbox_confs, axis=1)
 
+    box.stop_gradient = True
+    var.stop_gradient = True
     return mbox_locs_concat, mbox_confs_concat, box, var