Get the indexes of the layers where the size of the activation changes.

def _get_sfs_idxs(sizes:Sizes) -> List[int]:
    "Get the indexes of the layers where the size of the activation changes."
    feature_szs = [size[-1] for size in sizes]
    sfs_idxs = list(np.where(np.array(feature_szs[:-1]) != np.array(feature_szs[1:]))[0])
    if feature_szs[0] != feature_szs[1]: sfs_idxs = [0] + sfs_idxs
    return sfs_idxs