Minor improvements in Sentinel-2 Inference Example

[robmarkcole]

Following https://github.com/allenai/satlas/issues/13 I note a couple of minor improvements

• from osgeo import gdal is not required for this example and can be removed (somehow this always gives headaches on mac)
• device = torch.device("cuda") -> preferable to use device = torch.device("cuda" if torch.cuda.is_available() else "cpu") to avoid errors on CPU

[favyen2]

Thanks, made these changes (updated the device in the other scripts as well)