Update default trainer configuration parameters for improved training stability

[gitttt-1234]

Summary

This PR updates the default values for learning rate scheduler and optimizer configurations to improve training performance and stability based on empirical testing and best practices.

Configuration Changes

OptimizerConfig

• Learning rate: 1e-3 → 1e-4
  • More conservative initial learning rate for better convergence

ReduceLROnPlateauConfig

• threshold_mode: "rel" → "abs"
  • Absolute threshold mode provides more consistent behavior across different loss scales
• threshold: 1e-4 → 1e-6
  • Finer-grained sensitivity to loss improvements
• patience: 10 → 5
  • Faster adaptation to plateaus
• factor: 0.1 → 0.5
  • More gradual learning rate reduction
• cooldown: 0 → 3
  • Prevents oscillations after LR reduction
• min_lr: 0.0 → 1e-8
  • Ensures learning rate doesn't drop to zero

EarlyStoppingConfig

• min_delta: 0.0 → 1e-8
  • More forgiving threshold for improvement
• patience: 1 → 10
  • Allows more time for convergence before stopping

LRSchedulerConfig

• Now defaults to ReduceLROnPlateauConfig instead of None
  • Enables learning rate scheduling by default for better training dynamics

Files Updated

• ✅ sleap_nn/config/trainer_config.py - Updated defaults and documentation
• ✅ All sample config files in docs/sample_configs/ (11 files)
• ✅ All test config files in tests/assets/model_ckpts/ (12 files)
• ✅ Configuration documentation in docs/config.md
• ✅ Test assertions in tests/config/test_trainer_config.py

Benefits

• 🎯 More conservative and stable training behavior
• 📉 Better handling of loss plateaus with absolute threshold mode
• ⏱️ Improved early stopping behavior with reasonable patience
• 🔄 Learning rate scheduling enabled by default

Testing

• ✅ All tests pass (uv run pytest .)
• ✅ Linter passes (uv run ruff check sleap_nn/)
• ✅ Updated test assertions to match new defaults

Backwards Compatibility

These changes update default values only. Users with existing configurations will continue to use their specified values. The new defaults provide better out-of-box performance for new users and projects.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.83%. Comparing base (ff91433) to head (d62cd78).
⚠️ Report is 64 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #374      +/-   ##
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- Coverage   95.28%   93.83%   -1.46%     
==========================================
  Files          49       49              
  Lines        6765     7181     +416     
==========================================
+ Hits         6446     6738     +292     
- Misses        319      443     +124     

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