Fold constant_pad_nd into convolution input padding

[JakeStevens]

Summary:
When using padding="same" for a conv with even kernels, torch.export creates an explicit pad node, leading to a dq -> pad -> conv chain, which is not recognized, so conv ends up undelegated. The ReLU ends up in its own partition with a quantize. XNNPACK does not support this, resulting in xnn_status_unsupported_parameter.

This PR fixes this, allowing for full delegation to XNNPACK in this case, by pulling the pad into the same partition as the conv, and then re-folding it back into the conv itself in the XNNPACK input padding metadata, as XNNPACK does support this.

Differential Revision: D109871964

[pytorch-bot]

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