Henryh/pre train tutorial

[Hgherzog]

Depends on #393
Major Documentation Overhaul
New comprehensive documentation structure:

• docs/Pretraining.md - Complete pretraining guide covering:
• Environment setup for both external users and AI2 researchers
• Script launching with torchrun and command structure
• Dataset setup (H5 format requirements and structure)
• Official training scripts table (nano/tiny/base/large)
• Configuration overrides and experiment customization
• Hardware adaptation notes
• docs/Setup-Internal.md - AI2-specific guide:
• Beaker setup (GitHub tokens, workspace/budget config, secrets)
• Launch methods (pre-emptible jobs vs interactive sessions)
• Internal dataset locations on Weka
• Beaker gotchas and best practices

README.md cleanup:

• Removed detailed "Training Setup", "Launch", and "Beaker Information" sections (now in dedicated docs)
• Deleted beaker_config_example.yaml

Enabling Pre-training to Run not on Beaker

• Dataset path Centralization:
  • New olmoearth_pretrain/evals/datasets/paths.py centralizes all eval dataset paths
  • Supports environment variable overrides (e.g., GEOBENCH_DIR, PASTIS_DIR)
  • Removed hardcoded *_DIR constants from individual dataset modules (breizhcrops, cropharvest, floods, mados, pastis, geobench)
• Allow defaulting to not using beaker based env vars if not running on beaker

New Official Training Scripts

• Added scripts/official/ with model size variants:
• nano.py + nano_launch.sh - 4 GPU experiments with 9 lr/wd combinations
• tiny.py + tiny_launch.sh - 4 GPU experiments with 9 lr/wd combinations
• base.py + base_launch.sh - 8 GPU experiments with 9 lr/wd combinations
• large.py + large_launch.sh - Large model experiments with 9 lr/wd combinations
• ablations/base_mae.py - MAE ablation configuration

Impact
This PR makes the codebase significantly more accessible to external users while maintaining internal AI2 workflows. The documentation now has separation between internal/external use cases, and the code is better organized with centralized configuration and optional dependencies

[favyen2]

leaving a couple comments for now, didn't have time to go over the whole thing