More fixes from the max-effort review, after verifying each against the C++
reference and reachability:

- graph_copy _relayout_args: ranges picked the consumer iteration shape by rank
  alone (max key=len), so for two equal-rank operands with different per-dim
  extents the broadcast-from operand's smaller shape could win and the real
  incompatible-radix conflict on the broadcast-to dim was missed (order-dependent:
  a commutative reorder flipped correct relayout into a silent miss). Use per-dim
  max extent over the max-rank operands.

- lower_to_vcix _sew/_legalize_vector_type: mirror the C++ legalizeVectorType --
  F16/BF16 return sew 0 (transcendentals stay unlowered for -convert-math-to-llvm,
  as in the validated path) instead of being lowered to VCIX, and add the missing
  rank != 1 guard.

- lower_to_vcix matmul: port the C++ guards as loud failures -- M/N/K must be a
  multiple of the systolic size when > SS (else the N//SS / K//SS loops drop the
  tail tile), and A vs B must agree on the K subtile (last-writer-wins would pick
  one silently). Latent today (heuristic/autotune only emit SS-multiple tiles).

- Doc-only: graph-copy is default-on (TORCHSIM_GRAPH_COPY=0 to disable); fixed the
  two stale 'no-op unless set' comments.

Validated: elementwise, gemm, bmm, conv2d, group_conv, cat, floor/mod suite,
softmax, layernorm -- all pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

…fix exp chunk

Two fixes to the C++->Python vcix port (lower_to_vcix.py) that SDPA exercises but
the gemm/bmm/conv tests do not:

- _lower_matmul bailed with 'if ATag is None or BTag is None: return False',
  gating on an MVIN dma_start tag for both operands. In SDPA's fused scores.V
  matmul, operand B is the softmax output produced in place by affine.vector_store,
  not DMAed, so BTag stayed None and the matmul was left un-lowered -> wrong
  attention output. Mirror the C++ MatmulOpLowering: an operand is initialized by
  either a dma_start OR a preceding affine.vector_store into its root memref; bail
  only when an operand is truly uninitialized. BTag/BAsync stay None/0 and are only
  read under 'if BAsync:', so the B dma_wait is correctly skipped (as in C++).

- _make_sf_vc_v_iv n>1 transcendental chunking called
  vector.ExtractStridedSliceOp(offsets, sizes, strides, vec) -- wrong arg order,
  missing the result type and vector operand, raising TypeError under these MLIR
  bindings. Pass (result=legal_ty, vector=vec, offsets, sizes, strides). Only
  reached by large transcendentals (n>1), e.g. SDPA softmax exp, so CI's small-tile
  (n==1) tests never hit it.

Validated end-to-end (Spike+TOGSim allclose): SDPA 56 cases pass (was crash/wrong);
matmul/bmm/conv2d regress clean. Bisected: C++ vcix passes SDPA, Python vcix did
not; exp chunking and fine-grained ruled out separately.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>