Replace Conv3d pos_conv with equivalent Conv2d (Safari/Metal WebGPU compat)

unet.onnx

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
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-size 906555486
+oid sha256:e3f90f52f72378339b990459fadb29a68d3c7b5c6851545ba42774f489160b08
+size 906698976