Rectangular SIP topology + 6-device allreduce sweep

mesh_2d, torus_2d, and mesh_2d_no_wrap accept optional w,h kwargs;
sqrt fall-back preserved for square layouts (back-compat tests
confirm 4-SIP and 9-SIP square configs still work). sfr_config
reads system.sips.w/h from spec and threads dims through to the
topology fn.

test_allreduce_multidevice CONFIGS switched from 4 SIPs (square)
to 6 SIPs: ring_1d_6sip, torus_2d_6sip_2x3, mesh_2d_no_wrap_6sip_2x3.
_write_temp_configs writes system.sips.w/h when supplied;
_sip_topo_dims reads them back. Latency sweep loop also moved to
6-SIP layouts. Linear-scale plot variants dropped -- only log-scale
*.png + summary.csv emitted. Plots in tests/allreduce_latency_plots
regenerated.

New tests/test_sip_topology_rectangular.py asserts neighbor
correctness for 2x3 layouts and back-compat for square fallback.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_sip_topology_rectangular.py

@@ -0,0 +1,106 @@
+"""Rectangular (non-square) SIP-level 2D topology support.
+
+Phase 1 regression target: today the 2D builtin topology functions in
+``kernbench.ccl.topologies`` (``mesh_2d``, ``torus_2d``,
+``mesh_2d_no_wrap``) hardcode ``side = sqrt(world_size)`` and raise
+``ValueError`` for any non-square ``world_size``. This blocks running
+the allreduce sweep at n_sips=6 on torus/mesh layouts.
+
+Phase 2 will extend these functions to accept optional ``w, h`` kwargs
+so a 2×3 (or 3×2, etc.) layout works. Until then, every test below is
+expected to FAIL.
+
+Layout convention used here (matches non-rectangular case):
+    rank = row * w + col   for 0 <= row < h, 0 <= col < w
+
+For w=2, h=3, world_size=6 the layout is:
+
+         col=0  col=1
+  row=0:   0      1
+  row=1:   2      3
+  row=2:   4      5
+"""
+from __future__ import annotations
+
+import pytest
+
+from kernbench.ccl.topologies import (
+    mesh_2d,
+    mesh_2d_no_wrap,
+    torus_2d,
+)
+
+
+# ── mesh_2d_no_wrap (no wrap-around) ──────────────────────────────────
+
+
+def test_mesh_2d_no_wrap_2x3_top_left():
+    """rank 0 (top-left, no N, no W): only S and E."""
+    nbrs = mesh_2d_no_wrap(rank=0, world_size=6, w=2, h=3)
+    assert nbrs == {"S": 2, "E": 1}, nbrs
+
+
+def test_mesh_2d_no_wrap_2x3_top_right():
+    """rank 1 (top-right, no N, no E): only S and W."""
+    nbrs = mesh_2d_no_wrap(rank=1, world_size=6, w=2, h=3)
+    assert nbrs == {"S": 3, "W": 0}, nbrs
+
+
+def test_mesh_2d_no_wrap_2x3_middle_left():
+    """rank 2 (middle-left, no W): N, S, E."""
+    nbrs = mesh_2d_no_wrap(rank=2, world_size=6, w=2, h=3)
+    assert nbrs == {"N": 0, "S": 4, "E": 3}, nbrs
+
+
+def test_mesh_2d_no_wrap_2x3_bottom_right():
+    """rank 5 (bottom-right, no S, no E): only N and W."""
+    nbrs = mesh_2d_no_wrap(rank=5, world_size=6, w=2, h=3)
+    assert nbrs == {"N": 3, "W": 4}, nbrs
+
+
+# ── torus_2d (wrap-around on all four edges) ─────────────────────────
+
+
+def test_torus_2d_2x3_top_left():
+    """rank 0: N wraps to row 2 col 0 (rank 4); W wraps to col 1 (rank 1)."""
+    nbrs = torus_2d(rank=0, world_size=6, w=2, h=3)
+    assert nbrs == {"N": 4, "S": 2, "W": 1, "E": 1}, nbrs
+
+
+def test_torus_2d_2x3_bottom_right():
+    """rank 5: S wraps to row 0 (rank 1); E wraps to col 0 (rank 4)."""
+    nbrs = torus_2d(rank=5, world_size=6, w=2, h=3)
+    assert nbrs == {"N": 3, "S": 1, "W": 4, "E": 4}, nbrs
+
+
+# ── mesh_2d alias for torus_2d ───────────────────────────────────────
+
+
+def test_mesh_2d_2x3_matches_torus_2d():
+    """mesh_2d is currently a torus alias; behaviour must match torus_2d."""
+    for rank in range(6):
+        assert mesh_2d(rank=rank, world_size=6, w=2, h=3) == \
+            torus_2d(rank=rank, world_size=6, w=2, h=3)
+
+
+# ── Back-compat: square layouts still work without w/h kwargs ────────
+
+
+def test_square_back_compat_mesh_2d_no_wrap():
+    """Calling without w, h should still work for square world_size."""
+    nbrs = mesh_2d_no_wrap(rank=0, world_size=4)
+    assert nbrs == {"S": 2, "E": 1}, nbrs
+
+
+def test_square_back_compat_torus_2d():
+    nbrs = torus_2d(rank=0, world_size=4)
+    assert nbrs == {"N": 2, "S": 2, "W": 1, "E": 1}, nbrs
+
+
+# ── Validation: w*h must match world_size ────────────────────────────
+
+
+def test_rectangular_dims_must_match_world_size():
+    """Phase 2 contract: explicit w, h must satisfy w*h == world_size."""
+    with pytest.raises(ValueError):
+        mesh_2d_no_wrap(rank=0, world_size=6, w=3, h=3)  # 9 != 6