Load a `Learner` object saved with `export_state` in `path/file` with empty data, optionally add `test` and load on `cpu`. `file` can be file-like (file or buffer)

def load_learner(path:PathOrStr, file:PathLikeOrBinaryStream='export.pkl', test:ItemList=None, **db_kwargs):
    "Load a `Learner` object saved with `export_state` in `path/file` with empty data, optionally add `test` and load on `cpu`. `file` can be file-like (file or buffer)"
    source = Path(path)/file if is_pathlike(file) else file
    state = torch.load(source, map_location='cpu') if defaults.device == torch.device('cpu') else torch.load(source)
    model = state.pop('model')
    src = LabelLists.load_state(path, state.pop('data'))
    if test is not None: src.add_test(test)
    data = src.databunch(**db_kwargs)
    cb_state = state.pop('cb_state')
    clas_func = state.pop('cls')
    res = clas_func(data, model, **state)
    res.callback_fns = state['callback_fns'] #to avoid duplicates
    res.callbacks = [load_callback(c,s, res) for c,s in cb_state.items()]
    return res