[Bug] [Relax][ONNX] Resize produces wrong results with half_pixel/pytorch_half_pixel coordinate transformation and non-integer scales

[wuyii8941]

Expected behavior

Resize with coordinate_transformation_mode=half_pixel (or pytorch_half_pixel, asymmetric) and non-integer scale factors (e.g., 2.5x) should compute source coordinates as:

src = (dst + 0.5) / scale - 0.5

and produce results matching ONNX Runtime.

Actual behavior

TVM computes incorrect source coordinates, leading to wrong pixel mapping. For a 3×3 input with scale=2.5, the max absolute difference vs ORT is 4.0 (nearest mode) and 0.63 (linear mode).

Reproduction

import numpy as np
import onnx
from onnx import helper, TensorProto, numpy_helper
import onnxruntime as ort
import tvm
from tvm import relax
from tvm.relax.frontend.onnx import from_onnx

x = np.arange(9, dtype=np.float32).reshape(1, 1, 3, 3)

X = helper.make_tensor_value_info("X", TensorProto.FLOAT, [1, 1, 3, 3])
Y = helper.make_tensor_value_info("Y", TensorProto.FLOAT, None)
roi = numpy_helper.from_array(np.array([], dtype=np.float32), "roi")
sc = numpy_helper.from_array(np.array([1, 1, 2.5, 2.5], dtype=np.float32), "scales")
node = helper.make_node("Resize", ["X", "roi", "scales"], ["Y"],
                        mode="nearest",
                        coordinate_transformation_mode="half_pixel",
                        nearest_mode="floor")
graph = helper.make_graph([node], "test", [X], [Y], initializer=[roi, sc])
model = helper.make_model(graph, opset_imports=[helper.make_opsetid("", 18)])
model = onnx.shape_inference.infer_shapes(model)

# ORT
sess = ort.InferenceSession(model.SerializeToString())
ort_out = sess.run(None, {"X": x})[0]

# TVM
mod = from_onnx(model)
exe = tvm.relax.build(
    tvm.ir.transform.Sequential([relax.transform.LegalizeOps()])(mod), target="llvm")
vm = tvm.relax.VirtualMachine(exe, device=tvm.cpu())
tvm_out = vm["main"](tvm.runtime.tensor(x, device=tvm.cpu())).numpy()

print("Max diff:", np.abs(ort_out - tvm_out).max())  # 4.0

Affected configurations

mode	coordinate_transformation_mode	max_diff vs ORT
nearest	half_pixel	4.0
nearest	pytorch_half_pixel	4.0
linear	half_pixel	0.63
linear	asymmetric	0.46
linear	pytorch_half_pixel	0.63

Trigger condition: non-integer scale factor (e.g., 2.5) on odd spatial dims (e.g., 3×3). Integer scales (2x, 3x) appear correct.

Root cause

The coordinate transformation for half_pixel mode applies floor((dst + 0.5) / scale - 0.5) but the implementation appears to use a different rounding or offset, causing the source index to be off by one pixel for certain output positions.

Environment

• TVM: 0.24.dev0, commit 0b0afd8 (2026-04-24)
• Python: 3.11
• OS: Linux

cc @KJlaccHoeUM9l @junrushao