deal with fitting stimuli for LogitsBehavior (imagenet)

[mschrimpf]

The current LogitsBehavior implementation assumes that the fitting_stimuli will just be the string "imagenet".
We should remove this restriction and instead fit a linear classifier to the features in response to the fitting stimuli (see brain-score/vision#262).

[mschrimpf]

problem with this: we would have to provide the ImageNet training set for classifier fitting which is 140G. So likely we cannot provide it via BrainIO but have to point to it via direct filepath.