Hi,

In the new Complex and semantic figure composition (https://matplotlib.org/3.3.0/tutorials/provisional/mosaic.html) system one can give a global `subplot_kw`

where you define the projection of the axes of the figure.

For example from the above documentation

```
axd = plt.figure(constrained_layout=True).subplot_mosaic(
"AB", subplot_kw={"projection": "polar"}
)
```

Is there a way you can give mupltiple projections (for example polar and cartesian) for a figure? [Probably by providing the subplot_kw with a dictionary]

Thanks!