Split a RNN `model` in groups for differential learning rates.

def tfmerXL_lm_split(model:nn.Module) -> List[nn.Module]:
    "Split a RNN `model` in groups for differential learning rates."
    encoder = model[0]
    n = len(encoder.layers)//3
    groups = [list(encoder.layers[:n]) + [ParameterModule(encoder.u), ParameterModule(encoder.v)]]
    return groups + [list(encoder.layers[n:2*n]), list(encoder.layers[2*n:]), [encoder.encoder, model[1]]]