Add OlmoEarth as a supported backbone

[whatnick]

Feature Request

Description
Add support for OlmoEarth as a backbone in TerraTorch, enabling fine-tuning for downstream geospatial tasks (segmentation, classification, pixel-wise regression).

Motivation
OlmoEarth is an open-source Earth system foundation model from Allen AI that achieves strong performance on Earth observation tasks. It would be a valuable addition to TerraTorch's set of supported GFM backbones alongside Prithvi, TerraMind, Clay, etc.

References

• Repository: https://github.com/allenai/olmoearth_pretrain
• Paper: https://arxiv.org/abs/2511.13655

Suggested scope

• Register OlmoEarth as a backbone in the TerraTorch backbone registry
• Support combining it with existing decoders (UPerNet, FCN, etc.) via model factories
• Handle band/channel mapping for Sentinel-2 and other EO inputs