Update map_utils.py (#788)

* Update map_utils.py

The calculation may be not correct when the input bbox is not ordered by prob.
This commit fixes the problem.

ppdet/utils/map_utils.py

@@ -105,7 +105,8 @@ class DetectionMAP(object):
 
         # record class score positive
         visited = [False] * len(gt_label)
-        for b in bbox:
+        score_idx = np.argsort(-bbox[:, 1])
+        for b in bbox[score_idx, :]:
             label, score, xmin, ymin, xmax, ymax = b.tolist()
             pred = [xmin, ymin, xmax, ymax]
             max_idx = -1