I used hook WITHOUT removing/deleting it post training. Now when I am trying to re-run the notebooks I am getting OOM error as soon as I start training my first epoch.
Added .remove() in hooks now, but still not getting past the training. How should I free the CUDA memory? Tried resetting the runtime - which is not working! 
layers_ = flatten_model(learn.model)
class Hook():
def __init__(self, m, f): self.hook = m.register_forward_hook(partial(f, self))
def remove(self): self.hook.remove()
def __del__(self): self.remove()
def append_stats(hook, mod, inp, outp):
if not hasattr(hook,'stats'): hook.stats = ([],[],[])
means,stds, outs = hook.stats
if mod.training:
means.append(outp.data.mean())
stds .append(outp.data.std())
outs.append(outp)
`