Fix generic im2row NHWC layout to match Python reference

[ethansfng]

Summary:
The generic Cadence im2row kernel wrote its NHWC (channel_last=True) output in kernel-position-major order [kp][c], but the operator contract — defined by the Python reference in ref_implementations.py via torch.nn.functional.unfold — is channel-major [c][kp], i.e. column index c*(kH*kW) + kh*kW + kw. The conv-lowering pass ReplaceConvWithIm2RowAndLinear packs the matmul weights in the same [c][kp] order (permute [OC,kH,kW,IC] -> [OC,IC,kH,kW] -> [OC,K]), so the generic kernel's [kp][c] output was transposed relative to the weights.

This rewrites the generic NHWC branch to write [c][kp] (per-channel scatter data_col[i_col*channels_col + c*num_kp + kp])

Differential Revision: D110508326

[pytorch-bot]

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❌ 1 New Failure

As of commit 2312458 with merge base 8965e51 ():

NEW FAILURE - The following job has failed:

• pull / test-binary-size-linux-gcc / linux-job (gh)
  RuntimeError: Command docker exec -t fbc1455cb92f9f089e76d1663b03e5c977cb313189e374b1bf23c6dadb2c74de /exec failed with exit code 1

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[meta-codesync]

@ethansfng has exported this pull request. If you are a Meta employee, you can view the originating Diff in D110508326.

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