some pictures in annotation txt（voc2007.txt+ voc2012.txt） are not in the image folder（2007trainval + 2012trainval）

[Tb2019]

eg：009963.jpg is in the voc2007.txt as a train image's name but it is in the voc2007test folder as a test image actually.so the dataloder（getitem）can not load the picture,and will cause error --'NoneType' object has no attribute 'shape' during trainning the net.