fix error while loading cv2.VideoCapture (#2344)

* test=develop fix error while loading cv2.VideoCapture

* test=develop
fix bug of index out of range in SampleFrames

* fix bug

PaddleCV/video/datareader/kinetics_reader.py

@@ -14,6 +14,7 @@
 
 import os
 import sys
+import cv2
 import math
 import random
 import functools
@@ -386,7 +387,6 @@ def video_loader(frames, nsample, seglen, mode):
 def mp4_loader(filepath, nsample, seglen, mode):
     cap = cv2.VideoCapture(filepath)
     videolen = int(cap.get(cv2.CAP_PROP_FRAME_COUNT))
-    average_dur = int(videolen / nsample)
     sampledFrames = []
     for i in range(videolen):
         ret, frame = cap.read()
@@ -395,7 +395,7 @@ def mp4_loader(filepath, nsample, seglen, mode):
             continue
         img = frame[:, :, ::-1]
         sampledFrames.append(img)
-
+    average_dur = int(len(sampledFrames) / nsample)
     imgs = []
     for i in range(nsample):
         idx = 0
@@ -417,7 +417,7 @@ def mp4_loader(filepath, nsample, seglen, mode):
                 idx = i
 
         for jj in range(idx, idx + seglen):
-            imgbuf = sampledFrames[int(jj % videolen)]
+            imgbuf = sampledFrames[int(jj % len(sampledFrames))]
             img = Image.fromarray(imgbuf, mode='RGB')
             imgs.append(img)