bgv: add lowering to poly about heir HOT 3 OPEN

I'm having difficulty with lowering ModulusSwitch, I was trying to lower directly to arith, but faced avalanche of complications (casts, signedness, overflows).
Also it feels idiomatically incorrect to lower straight to arith, bypassing poly.

@asraa , @j2kun , Would it be reasonable to have a Poly_ScaleOp that takes poly argument and float (either argument or attribute?) and returns element-wise multiplied poly (rounded down)?

from heir.

What do you think about waiting until we have a unified LWE/RLWE dialect? I agree that lowering to arith is weird, but if you want to go through poly now, you could lower to a combination of to_tensor and from_tensor, using tensor/math/arith ops for the rescaling and rounding.

from heir.

you could lower to a combination of to_tensor and from_tensor, using tensor/math/arith ops for the rescaling and rounding.

That what I'm currently doing, so output is still in poly. My concern was about "going level too deep" during lowering.

What do you think about waiting until we have a unified LWE/RLWE dialect?

I think it will pose the same problem. I will proceed with crude lowering approach and send out PR to gather concrete feedback.

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