Fix for AP calculation limits 0.0 - 1.0 (#4563)

This PR brings alignment in AP computation practices with Detectron2 and MMDetection. 

Problem first noted by @yusiyoh in ultralytics/yolov5#4546

utils/metrics.py

@@ -91,8 +91,8 @@ def compute_ap(recall, precision):
     """
 
     # Append sentinel values to beginning and end
-    mrec = np.concatenate(([0.], recall, [recall[-1] + 0.01]))
-    mpre = np.concatenate(([1.], precision, [0.]))
+    mrec = np.concatenate(([0.0], recall, [1.0]))
+    mpre = np.concatenate(([1.0], precision, [0.0]))
 
     # Compute the precision envelope
     mpre = np.flip(np.maximum.accumulate(np.flip(mpre)))