Add LL::quotient and remove uses of divideRight and sublayoutIsIdentity

[lezcano]

We add a new abstraction LL::quotient that abstracts the idea of "a
linear layout does not permute certain dimensions". Doing so, allows us
to remove divideRight and subsume them into this higher-level abstraction.
We also fix a bug in isCrossCTAConversion.

We also remove some code duplication from transferWithinThreads and
cvtReorderRegisters in favour of a more generic approach.

We fix a bug in sublayout that meant that sublayout would reorder outDims
at will by using a set instead of a vector.

I am missing adding tests for LL::quotient, will do in a minute.

[Jokeren]

May I know what bug is there?

We also fix a bug in isCrossCTAConversion.

[Jokeren]

So, are we ignoring the case where the input dimension names are not equal to the output dimension names, but the input and output are identity mappings?

[lezcano]

The identity in maths is just defined between a space and itself, so yes, this is the correct mathematical concept. Even if you have registers that map 1-to-1 to a vector of outputs, this would not be an identity map strictly speaking, as it's not mapping elements to themselves, but identifying registers with a matrix.

[Jokeren]

OK, makes sense then

[lezcano]

May I know what bug is there?

Previously, we were checking that:

return !layout.sublayoutIsZero(nonBlockInDims.getArrayRef(), {kBlock}) ||
         !layout.sublayoutIsIdentity({kBlock}, {kBlock});

This means that the kBlock of the output will be defined uninquely and as the identity by the kBlock of the input, which is fine. But we are missing a check that layout.sublayoutIsZero({kBlock}, nonBlockOutDims.getArrayRef()), as otherwise the layout will depend on the kBlock, but a few lines after that we are "removing" the kBlock dimensions, effectively assuming that that sublayout is zero.

At the moment I have drawn this rather rough distribution, but it can be optimised in the future. For example, if you just have those two initial conditions, you can still perform the conversion without using distributed shmem, but you'll need to write an indexing that depends on kBlock, which is a tad trickier, but of course, can be done.

[Jokeren]

This means that the kBlock of the output will be defined uninquely and as the identity by the kBlock of the input, which is fine. But we are missing a check that layout.sublayoutIsZero({kBlock}, nonBlockOutDims.getArrayRef()), as otherwise the layout will depend on the kBlock, but a few lines after that we are "removing" the kBlock dimensions, effectively assuming that that sublayout is zero.

Sure, I think it makes sense