bug: ng_run with no config spawns Ray and hangs instead of failing fast #1489

Summary

Found by our virtual developer review agent.

When +config_paths is omitted (or resolves to an empty list), ng_run proceeds to spin up Ray and the head server with zero configured servers. The process never converges and is eventually killed with a native SIGTERM dump, leaving the user with low-level signal noise instead of a clear message that a config is required.

Location

nemo_gym/global_config.py — config parsing (~line 411)

config_paths = merged_config_for_config_paths.get(CONFIG_PATHS_KEY_NAME) or []
config_paths = ta.validate_python(config_paths)
config_paths, extra_configs = self.load_extra_config_paths(config_paths)

There is no validation that at least one config path is provided before downstream startup.

Current behavior

ng_run

Ray cluster and head server start with zero servers. The run hangs until killed externally, producing output like:

logging.cc: *** SIGTERM received ... AbslFailureSignalHandler() ... kevent

No message indicates that +config_paths is required.

Expected behavior

Fail fast before any Ray or head-server startup with a clear message, similar to the guard already used in ng_prepare_benchmark:

assert benchmarks_dict, (
    'No benchmark config found in config_paths. Pass a benchmark config, e.g.: "+config_paths=[benchmarks/aime24/config.yaml]"'
)

Suggested fix

In parse() or early in run() (before RunHelper.start()), check whether resolved config_paths is empty after parsing. If so, raise a clean SystemExit:

No config provided. Pass +config_paths=[...]. See ng_help.

This check must happen before Ray or head-server startup.

Repro

ng_run
# or explicitly:
ng_run "+config_paths=[]"