Store win/hop duration for outputs that return frames

[hagenw]

When inspecting a loaded model it is possible to have a dimension that has no fixed size, e.g.

  cnn_features:
    shape: [1, 512, -1]
    dtype: tensor(float)
    labels: [cnn_features-0, cnn_features-1, cnn_features-2, (...), cnn_features-509,
      cnn_features-510, cnn_features-511]

In order to be able to use such a model with audinterface one needs to know the hop duration (and window duration) of the output, so it would be nice if this would be stored as part of the model metadata.