Add --load-tasks-multilingual and fix --custom-tasks for inspect backend

[dzautner]

Summary

The inspect backend (lighteval eval) hardcodes custom_tasks=None and load_multilingual=False when constructing the Registry, making it impossible to use multilingual or custom tasks. All other backends (vllm, accelerate, sglang, endpoint) properly accept and pass through both parameters.

This PR:

• Adds --load-tasks-multilingual CLI flag to the inspect backend, reusing the shared load_tasks_multilingual arg from cli_args.py (same pattern as all other backends)
• Fixes custom_tasks being hardcoded to None — the parameter already exists in the function signature but was never forwarded to Registry

Changes (3 lines in main_inspect.py)

1. Import the shared CLI arg definition
2. Add the load_tasks_multilingual parameter to the eval() function
3. Pass both custom_tasks and load_tasks_multilingual to Registry() instead of hardcoded values

Usage

lighteval eval "vllm/model" "mgsm:fi" --load-tasks-multilingual
lighteval eval "vllm/model" "my_task" --custom-tasks path/to/tasks.py

Test plan

• Verified --load-tasks-multilingual flag appears in CLI help
• Verified multilingual tasks load when flag is set (Registry finds tasks from multilingual/tasks/)
• End-to-end eval run with maime25:fi (30 samples) completed successfully on AMD MI325X cluster using vLLM backend through inspect-ai
• Ruff formatting passes