I tried to train on some of my USDZs. For some the CreateML viewport shows the object's dimensions and enables the Train button. Many others, including the official USDZ for the iPhone 15Pro (iphone_15_pro_blue_titanium_5G.usdz) do not show the dimensions and keep [Train] disabled.
What do I have to author inside the USDZ to make it work? There are no lights or cameras in the asset file, as recommended in the release notes. Also no animations.