`Byte_Representation` gadget may lead to vk change against each assignment

[noel2004]

In Byte_Representation gadget, the selector (a fixed col) is enabled while assigning byte represents:

mpt-circuit/src/gadgets/byte_representation.rs

Line 107 in 2cc63d2

self.selector.enable(region, offset);

It should be supposed that we have diffferent counts on assigned bytes for synctheis base on the witness. That is, the circuit (vk and pk) would change with changing on witness and the test vk_validity would expect to fail

[z2trillion]

Yes, this is problem now. I plan to solve this by padding this gadget (and all other gadgets) with default assignments to a fixed length.

For this gadget, assigning 0 to every advice column should satisfy the constraints.