Fix DequantizeLinear-21 blocked quantization for axis > 1

[evkotov]

Details:

This PR fixes incorrect behavior of DequantizeLinear operator (opset 21) with blocked quantization when axis > 1.

The old implementation had two issues:

1. Did not support axis > 1 (returned error "Axis > 1 isn't supported")
2. For axis = 1, produced incorrect results due to wrong dimension ordering

The root cause was incorrect reshape logic that used different ordering for axis = 0 vs axis > 0:

• axis = 0: [num_blocks, block_size, ...] - correct
• axis > 0: [D0, ..., block_size, num_blocks, ...] - incorrect

This caused elements to interleave instead of forming sequential blocks, which violates the ONNX specification.

Example

For axis=1, block_size=2, input=[9,9,9,9,9,9,9,9], scale=[1,2,3,4], zero_point=[1,2,3,4]:

• Old output (incorrect): [8, 14, 8, 14, 18, 20, 18, 20] - elements interleaved
• New output (correct): [8, 8, 14, 14, 18, 18, 20, 20] - sequential blocks
• ONNX Runtime: [8, 8, 14, 14, 18, 18, 20, 20] - matches new implementation

Changes

1. Removed axis <= 1 restriction
2. Fixed reshape to always use [D0, ..., num_blocks, block_size, ...] ordering for all axes
3. Fixed unsqueeze position: changed from conditional (axis == 0 ? 1 : axis) to always axis + 1

Implementation

src/frontends/onnx/frontend/src/op/dequantize_linear.cpp:

• Unified reshape logic: always split dimension at axis into [num_blocks, block_size]
• Fixed unsqueeze_axis = axis + 1 (previously was incorrect for axis > 0)
• Removed axis validation that limited support to axis <= 1

Updated 1 existing test:

• onnx_model_dequantize_linear_opset21_4d: fixed expected output (was using old incorrect interleaved values)

Tickets:

• 164793