update training-example with 0.4.1 and 0.5.0 releases

[roybenhayun]

the training-example is a valuable community resource for developers who want a reference working baseline for semantic segmentation.
at the moment it is ~2 releases behind torchgeo 0.4.1 and 0.5.0.

some notes:

• need to update pip installs and imports e.g., lightning
• del sample["bbox"] needs clarifications between 0.4.1 and 0.5.0
• SegmentationDataModule should extend GeoDataModule (see related discussion)
• params in SemanticSegmentationTask need update (may be different for 0.4.1 and 0.5.0)

valuable example. would be very very helpful to update.

see Adam's suggestion to open an issue.

FWIW, will be happy to interact, give feedback and testing. please feel free to contact directly.

[calebrob6]

Hey @roybenhayun, do you want to open a PR to update this?