task-based LR scheduling, finer-grained val reporting

[TimotheeMickus]

• [ ] break down accuracy and validation loss per task when reporting stats
• [ ] introduce validation-based LR scheduling (reduce LR on plateau) that's properly adjusted depending on the performances of individual modules:
  • for each module, initialize module-specific LRs to the default value
  • at validation:
    • compute perf per task
    • aggregate (mean or task-weighted mean) perf per module across task
    • reduce module-specific LR based on the aggregate perf