[DRAFT] Remove redundant adjoint GPU communication routines

[samhatfield]

In the CPU code path, we don't have "AD" versions for TRLTOG, TRLTOM etc. Instead we simply use the inverse routine (e.g. G->L instead of L->G) with the input / output arguments swapped. In the GPU code path though we have additional subroutines TRLTOMAD and TRMTOLAD. This PR is for investigating whether these routines can be eliminated. If this succeeds, then we can close PR #340 unmerged.

[samhatfield]

Any thoughts on this @l90lpa? I was hoping I could also delete TRLTOMAD_PACK_UNPACK and TRMTOLAD_PACK_UNPACK and somehow reuse the non-adjoint versions, saving another ~700 lines of code, but the adjoint packing/unpacking routines do seem to be genuinely different. I might therefore undo commit f995241.

[l90lpa]

Hi @samhatfield, sorry for the slow reply. Yes, I had originally wanted to take the approach you're discussing, however, the issue I ran into was that trltom_pack/trltom/trltom_unpack and trmtol_pack/trmtol/trmtol_unpack don't just do transposition but also have some over responsibilities built in that only apply to one transform direction. If I recall correct, trltom_pack isn't just packing a buffer, it is also applying quadrature weights. And again, if I recall correctly somewhere in trmtol_pack/unpack it handles frequency padding. There might be some other bits I forgot. At the time, I did look and is seemed possible to break up these routines to avoid needing additional adjoint code, but this would have required more work outside of the scope of the PRs than I had time for (and I wasn't sure if the GPU code had been written this way intentionally).