title: PyAV not installed error but it's installed (GoogleColab)
body: <p>I am trying to import the ucf101 dataset using</p>
  
  <pre><code>dataset = torchvision.datasets.UCF101('/content/drive/My Drive/Colab Notebooks/ucf101', annotation_path='/content/drive/My Drive/Colab Notebooks/ucfTrainTestlist', frames_per_clip=16, step_between_clips=1, frame_rate=None, fold=1, train=True, transform=transforms.Compose([transforms.ToTensor(),]))
  </code></pre>
  
  <p>but it gives me this error</p>
  
  <pre><code>    ImportError                               Traceback (most recent call last)
  &lt;ipython-input-8-2c7f10121439&gt; in &lt;module&gt;()
  ----&gt; 1 dataset = torchvision.datasets.UCF101('/content/drive/My Drive/Colab Notebooks/ucf101', annotation_path='/content/drive/My Drive/Colab Notebooks/ucfTrainTestlist', frames_per_clip=16, step_between_clips=1, frame_rate=None, fold=1, train=True, transform=transforms.Compose([transforms.ToTensor(),]))
  
  6 frames
  /usr/local/lib/python3.6/dist-packages/torchvision/datasets/ucf101.py in __init__(self, root, annotation_path, frames_per_clip, step_between_clips, frame_rate, fold, train, transform, _precomputed_metadata, num_workers, _video_width, _video_height, _video_min_dimension, _audio_samples)
       70             _video_height=_video_height,
       71             _video_min_dimension=_video_min_dimension,
  ---&gt; 72             _audio_samples=_audio_samples,
       73         )
       74         self.video_clips_metadata = video_clips.metadata
  
  /usr/local/lib/python3.6/dist-packages/torchvision/datasets/video_utils.py in __init__(self, video_paths, clip_length_in_frames, frames_between_clips, frame_rate, _precomputed_metadata, num_workers, _video_width, _video_height, _video_min_dimension, _video_max_dimension, _audio_samples, _audio_channels)
      116 
      117         if _precomputed_metadata is None:
  --&gt; 118             self._compute_frame_pts()
      119         else:
      120             self._init_from_metadata(_precomputed_metadata)
  
  /usr/local/lib/python3.6/dist-packages/torchvision/datasets/video_utils.py in _compute_frame_pts(self)
      140 
      141         with tqdm(total=len(dl)) as pbar:
  --&gt; 142             for batch in dl:
      143                 pbar.update(1)
      144                 clips, fps = list(zip(*batch))
  
  /usr/local/lib/python3.6/dist-packages/torch/utils/data/dataloader.py in __next__(self)
      343 
      344     def __next__(self):
  --&gt; 345         data = self._next_data()
      346         self._num_yielded += 1
      347         if self._dataset_kind == _DatasetKind.Iterable and \
  
  /usr/local/lib/python3.6/dist-packages/torch/utils/data/dataloader.py in _next_data(self)
      854             else:
      855                 del self._task_info[idx]
  --&gt; 856                 return self._process_data(data)
      857 
      858     def _try_put_index(self):
  
  /usr/local/lib/python3.6/dist-packages/torch/utils/data/dataloader.py in _process_data(self, data)
      879         self._try_put_index()
      880         if isinstance(data, ExceptionWrapper):
  --&gt; 881             data.reraise()
      882         return data
      883 
  
  /usr/local/lib/python3.6/dist-packages/torch/_utils.py in reraise(self)
      393             # (https://bugs.python.org/issue2651), so we work around it.
      394             msg = KeyErrorMessage(msg)
  --&gt; 395         raise self.exc_type(msg)
  
  ImportError: Caught ImportError in DataLoader worker process 0.
  Original Traceback (most recent call last):
    File "/usr/local/lib/python3.6/dist-packages/torch/utils/data/_utils/worker.py", line 178, in _worker_loop
      data = fetcher.fetch(index)
    File "/usr/local/lib/python3.6/dist-packages/torch/utils/data/_utils/fetch.py", line 44, in fetch
      data = [self.dataset[idx] for idx in possibly_batched_index]
    File "/usr/local/lib/python3.6/dist-packages/torch/utils/data/_utils/fetch.py", line 44, in &lt;listcomp&gt;
      data = [self.dataset[idx] for idx in possibly_batched_index]
    File "/usr/local/lib/python3.6/dist-packages/torchvision/datasets/video_utils.py", line 61, in __getitem__
      return read_video_timestamps(self.x[idx])
    File "/usr/local/lib/python3.6/dist-packages/torchvision/io/video.py", line 318, in read_video_timestamps
      _check_av_available()
    File "/usr/local/lib/python3.6/dist-packages/torchvision/io/video.py", line 40, in _check_av_available
      raise av
  ImportError: PyAV is not installed, and is necessary for the video operations in torchvision.
  See https://github.com/mikeboers/PyAV#installation for instructions on how to
  install PyAV on your system.
  
  ---------------------------------------------------------------------------
  NOTE: If your import is failing due to a missing package, you can
  manually install dependencies using either !pip or !apt.
  
  To view examples of installing some common dependencies, click the
  "Open Examples" button below.
  ---------------------------------------------------------------------------
  </code></pre>
  
  <p>The problem is that I have already installed and imported pyAV a few cells above</p>
  
  <pre><code>pip install av
  import av
  print (av.__version__)
  &gt;&gt;8.0.1
  </code></pre>