[Test] Pin CUDA round ties-to-even on exact midpoints

[LngelKyo]

test_vectorized_intrin1 exercises tvm.tirx.round on the CUDA path with np.random.uniform(0, 1) inputs and atol=1e-3, which cannot distinguish ties-to-even from ties-away-from-zero: exact midpoints (0.5, 1.5, 2.5, ...) never occur in random data. The CUDA rule lowers tirx.round to nearbyint / nearbyintf (src/backend/cuda/codegen/intrin_rule_cuda.cc:42-46, with the comment "Use nearbyint (ties-to-even) for round to match constant-folding semantics"), so the intended tie rule is already encoded — but nothing pinned it on this backend.

Scope follows the review on #20252, verbatim:

Please limit this test to the regression-relevant host targets (llvm and c) instead of enumerating every backend. Most of these device targets are skipped in CI, so this list adds maintenance cost without guaranteeing coverage. Backend-specific lowering should be tested in the corresponding codegen tests when needed.

A midpoint test in the CUDA codegen file is that instruction's second half.

What this test does

• Midpoint vector from [BugFix] Align tir.round to ties-to-even across all backends #19368 / [BugFix] Align default (C) tirx.round lowering to ties-to-even #20131 (0.5, 1.5, 2.5, 3.5, -0.5, -1.5, -2.5, -3.5), tiled ×16 to fill the 128 lanes sched() assumes (1 block × 32 threads × float4).
• Zero-tolerance check against np.round (rtol=0, atol=0). assert_allclose treats -0.0 and +0.0 as equal, so a separate np.signbit assertion pins that round(-0.5) must produce -0.0.
• Both compile paths (nvcc, nvrtc) are covered by the autouse fixture.
• Scope: float32 only. fp16/bf16 hrint is not covered here, consistent with test_vectorized_intrin1 skipping round on fp16.

Verification (my run): RTX A6000, driver 610.43.02, CUDA 13.0.88, LLVM 15.0.7, TVM at eaf710e on 5e49b34.

• test_round_ties_to_even[nvcc] / [nvrtc]: both PASSED.
• Negative control: reverting the CUDA rule to round/roundf makes both params FAIL, with 64 of 128 lanes off by one (0.5→1, 2.5→3, -0.5→-1, -2.5→-3). Restoring the rule turns both green again.
• uvx ruff@0.12.3 check and format --check clean.

cc @tlopex @yongwww @swjng — follows up on #20252 (CI will likely need approval as before).