[CMSIS-NN] Pad fusion with QNN Conv2D

[ashutosh-arm]

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Pass that fuses nn.pad and qnn.conv2d for CMSIS-NN target.

cc @Mousius @areusch @grant-arm @lhutton1 @manupa-arm

[lhutton1]

Thanks @ashutosh-arm, @SebastianBoblest!