…tion

This PR addresses issue google-deepmind#312 by updating all example scripts to use
lp_utils.launch_with_termination_handler() instead of lp.launch() directly.

The launch_with_termination_handler() function wraps lp.launch() with a signal
handler that ensures all Launchpad processes are properly terminated when the
parent process receives a SIGTERM or SIGINT signal. This prevents the messy
teardown messages like:
  'Worker groups that did not terminate in time: ['actor'] Killing entire runtime.'

This is especially important when using external loggers like Weights and Biases,
which previously reported experiments as 'crashed' even though they completed
successfully.

Files updated:
- examples/baselines/imitation/*.py
- examples/baselines/rl_continuous/*.py
- examples/baselines/rl_discrete/*.py
- examples/multiagent/multigrid/run_multigrid.py

Fixes google-deepmind#312