Add FindStadiums and GetImageTimeSeries examples

[favyen2]

• FindStadiums: shows how to fine-tune models when you only have point labels. Also shows how to compare OlmoEarth against other models like Clay and Prithvi.
• GetImageTimeSeries: shows how to use rslearn to get Sentinel-2 image series of some target locations.
• Fix bug in prithvi where positional embedding was being created on CPU for multi-temporal input

[favyen2]

Depends on #633