Add Pixio encoder

[xlei77]

Summary

• Register Pixio as a new encoder in the MapAnything model factory
• Add config files for the Pixio encoder and model (configs/model/encoder/pixio.yaml,
  configs/model/pixio.yaml)
• Wire up the Pixio entry in MODEL_CONFIGS in mapanything/models/init.py

Changes

• configs/model/encoder/pixio.yaml — New encoder config
• configs/model/pixio.yaml — New model config
• mapanything/models/init.py — Add "pixio" entry to MODEL_CONFIGS

Test plan

• Run inference with the Pixio model config end-to-end together with adding Pixio in UniCeption PR19