[neilmsft]

Removes the optional use_int8_per_channel re-encoding path from
MatMulNBitsToQDQ, retaining conversion that preserves the original
quantized weights instead of requantizing them to INT8.

Includes regression coverage for blockwise and per-channel weight
preservation, plus a guard that leaves nodes with unsupported
floating-point zero points unchanged.

[Copilot]

🟢 Approval recommended

No active review issues remain for the change removing optional INT8 re-encoding while retaining weight-preserving conversion.

No open issues 🎉

What changed in this PR

This PR removes optional INT8 per-channel re-encoding from Olive’s MatMulNBitsToQDQ pass, retaining conversion that preserves the original quantized weights.

Changes:

• Removes the use_int8_per_channel option and its re-encoding helper.
• Leaves nodes with floating-point zero points unchanged and logs a warning.
• Replaces re-encoding tests with regression coverage for weight preservation and floating-point zero-point handling.

File	Description
test/​passes/​onnx/​test_mnb_to_qdq.py	Tests blockwise and per-channel weight preservation, plus skipping unsupported zero points.
olive/​passes/​onnx/​mnb_to_qdq.py	Removes INT8 re-encoding and guards against floating-point zero-point conversion.

_{Review effort: Balanced}