add comments and docs. test=develop

Author: heavengate

paddle/fluid/operators/yolov3_loss_op.cc

@@ -98,7 +98,7 @@ class Yolov3LossOpMaker : public framework::OpProtoAndCheckerMaker {
              "This is a 4-D tensor with shape of [N, C, H, W]."
              "H and W should be same, and the second dimention(C) stores"
              "box locations, confidence score and classification one-hot"
-             "key of each anchor box");
+             "keys of each anchor box");
     AddInput("GTBox",
              "The input tensor of ground truth boxes, "
              "This is a 3-D tensor with shape of [N, max_box_num, 5], "
@@ -179,6 +179,11 @@ class Yolov3LossOpMaker : public framework::OpProtoAndCheckerMaker {
          box coordinates (w, h), and sigmoid cross entropy loss is used for box 
          coordinates (x, y), confidence score loss and classification loss.
 
+         Each groud truth box find a best matching anchor box in all anchors, 
+         prediction of this anchor box will incur all three parts of losses, and
+         prediction of anchor boxes with no GT box matched will only incur objectness
+         loss.
+
          In order to trade off box coordinate losses between big boxes and small 
          boxes, box coordinate losses will be mutiplied by scale weight, which is
          calculated as follow.

paddle/fluid/operators/yolov3_loss_op.h

@@ -308,13 +308,15 @@ class Yolov3LossKernel : public framework::OpKernel<T> {
               }
             }
 
+            // If best IoU is greater then ignore_thresh,
+            // ignore the objectness loss.
             if (best_iou > ignore_thresh) {
               int obj_idx = (i * mask_num + j) * stride + k * w + l;
               obj_mask_data[obj_idx] = static_cast<T>(-1);
             }
-            // TODO(dengkaipeng): all losses should be calculated if best IoU
-            // is bigger then truth thresh should be calculated here, but
-            // currently, truth thresh is an unreachable value as 1.0.
+            // all losses should be calculated if best IoU
+            // is bigger then truth thresh, but currently,
+            // truth thresh is an unreachable value as 1.0.
           }
         }
       }
@@ -341,8 +343,6 @@ class Yolov3LossKernel : public framework::OpKernel<T> {
           an_box.w = anchors[2 * an_idx] / static_cast<T>(input_size);
           an_box.h = anchors[2 * an_idx + 1] / static_cast<T>(input_size);
           float iou = CalcBoxIoU<T>(an_box, gt_shift);
-          // TODO(dengkaipeng): In paper, objectness loss is ignore when
-          // best IoU > 0.5, but darknet code didn't implement this.
           if (iou > best_iou) {
             best_iou = iou;
             best_n = an_idx;