Better way of determining output dimensionality in offline diags

[AnnaKwa]

Currently the offline diags use a CLI flag to determine if any model outputs are 3D. If so, pressure thickness is automatically added to the list of variables to load, and it is used to compute various 3D diagnostics with pressure as an axis. It would be nice if there was a way of determining a model's output dimensionality without having to manually specify this information or calling its predict method (which would require an additional data loading and prediction step in the diags).

[AnnaKwa]

As discussed with Brian, it should be ok to download and look at outputs in the first batch (even for C384) to do this check.