Pyro: scipy imresize is deprecated

[fehiepsi]

imresize is used in air example, but it is deprecated now. We need to replace it by something else to make tests pass.

More context about this issue: imresize is used in observations but this library is deprecated now in favour of tensorflow datasets. But multi-mnist is not available in tensorflow's datasets yet. So I'm not sure what is a good solution for this issue.

[neerajprad]

The problem is with the observations library (that uses this function) which is not being maintained anymore. I think we need to remove dependency on observations.

[null-a]

We need to replace it by something else

The docs suggest using numpy.array(Image.fromarray(arr).resize()) inplace of imresize. This requires PIL which AIR already has as a dependency.