Remove hooks once we're done with CAM extractors

[frgfm]

When an extractor is dereferenced, its hooks are not removed by default

extractor = LayerCAM(model)
del extractor
# Hooks are still on the model

A call to .remove_hooks is required to fix that.

What would avoid problems is to remove them when it's deleted:

extractor = LayerCAM(model)
del extractor
# No more hooks of that extractor on the model

but also when it's dereferenced

extractor = LayerCAM(model)
extractor = LayerCAM(model)
# No more hooks of the initial extractor on the model

An option would be to use a context manager

with LayerCAM(model) as extractor:
    # do stuff
# No more hooks on the model