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

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