Rearchitecture: Xe epilogue

[petercad]

This PR builds on #573, adding a CollectiveEpilogue with support for the new block 2D copy atoms.

The existing epilogue implementation was mostly rewritten, as it had many hardcoded assumptions and limitations:

• Subgroups own a contiguous tile within the workgroup tile
• Subgroup tiles are laid out n-major within the workgroup tile
• C/D atoms have the same block size
• One copy atom of data is processed at a time
• C/D atoms must bring data in the exact same layout as the accumulator

The new implementation removes all these restrictions.

Its API is also somewhat different, mostly in ways that more closely match the SM90 epilogues:

• Configurable EpilogueTile template parameter controls the block size for epilogue computation.
• Fusion callbacks receive workgroup-scope tiling information, not subgroup-scope tiling information (because CuTe's TiledMMA is very flexible -- the subgroup "tile" may not be contiguous).
• Vectorization for the epilogue compute operations is configurable via the ComputeVectorLen constexpr variable. Currently this is set to operate on one MMA atom's worth of accumulator data at a time, but if we want to make it user-configurable like the NV epilogues (where it's a template parameter for the dispatch policy), that's possible.
• It receives the TiledMMA as a template parameter rather than an argument to operator().
• The S2R/R2S copy operation parameters are omitted (a difference vs. SM90) as they are irrelevant to both the old and new epilogue implementation.

The new implementation glues together C/D loads and compute with reorders, so it can support efficient data type and layout conversions outside of the epilogue computation.