text: Title: Weakness: duplicated path concatenation in the toolkit/dataset/vot.py
  Body: As the title describes, there exists two times path concatenations for the member variable named `img_names` belonged to the `VOTVideo` class.
  One of them occurs at `line 18` in `toolkit/dataset/video.py` , and the other one is  at  `line 53` in `toolkit/dataset/vot.py` if the variable `load_img` is set to `False`.
  Here are some examples.
  
  absolute path
  ```Python
  >>> a = '/home/username/test_dir'
  >>> b = '/home/username/test_dir/test.py'
  >>> c = os.path.join(a, b)
  >>> print(c)
  /home/username/test_dir/test.py # valid path
  ```
  
  relative path
  ```Python
  >>> a = './test_dir'
  >>> b = './test_dir/test.py'
  >>> c = os.path.join(a, b)
  >>> print(c)
  ./test_dir/./test_dir/test.py # invalid path
  ```
  
  Consequently, if users pass a relative path to the `toolkit.datasets.DatasetFactory.create_dataset` method, their programs will crash. So, please fix this weakness timely.