Question about input_length in stsgcn()

[Annasong1010]

I'm wondering what's the reason that you recursively set the input_length (time steps) in each STSGC layer to minus 2. Does that mean you set T=12 in the first layer, T=10 in the second layer, T=8 in the third layer and T=6 in the fourth layer? Is that for best predictive accuracy or for better hierarchical spatial-temporal logics? Thank you.
(more specific, line 487 in stsgcn.py)