Add a helper function get_executor for retrieving executors from NEMORUN_HOME

[hemildesai]

Similar to workspace.py, but specifically for executors.

Example executors.py

import nemo_run as run

EXECUTOR_MAP = {
    "local": run.LocalExecutor()
}

Usage:

run.get_executor("local")

[marcromeyn]

What's the advantage over workspace.py? Could we combine the 2 somehow?

[hemildesai]

What's the advantage over workspace.py? Could we combine the 2 somehow?

This is specifically for executors. It can be used with/without CLI as per the user requirements. Maybe we can separate workspace.py into factories.py and executors.py to make it consistent across CLI/non-CLI.