lwe: add (optional) level and scale attributes to lwe.rlwe_encode

[AlexanderViand]

Preliminary for Cheddar work (where encoding a plaintext effectively requires knowing what level to encode it to, since the canonical scale is different for each level).

[j2kun]

The CI failure looks legit to me.

[AlexanderViand]

The CI failure looks legit to me.

Yep, looks like something broke when I cherry-picked it :(

[j2kun]

Can't the level be inferred from the result type of the encode op?

[AlexanderViand]

Can't the level be inferred from the result type of the encode op?

@j2kun Nope, unfortunately the info on the ptxt type isn't sufficient. In #3305 @ZenithalHourlyRate suggested also adding a ctxt space to the op as a solution, I instead picked this approach with an optional Attr when I needed it "just to make cheddar work" since its a lot lower-impact.

In the meantime, @mdgrs is looking into actually doing smart encoding-target-level selection in general, so maybe it is now actually worth doing a "real" solution rather than the optional attr workaround?

[j2kun]

I'm fine with this approach, though I suspect we will also want a scale attribute so that plaintext encoding can target a specific scale. Can you add that as another optional attribute, then we can get this in.

[AlexanderViand]

I'm fine with this approach, though I suspect we will also want a scale attribute so that plaintext encoding can target a specific scale. Can you add that as another optional attribute, then we can get this in.

@j2kun done :)