Support positional args in LazyEntrypoint._add_overwrite

[james77777778]

Hi team,

I found that nemo_run.cli.entrypoint doesn't support positional args when using CLI, but the docstrings from NeMo imply that it should:

https://github.com/NVIDIA/NeMo/blob/d82f53a23d013cfe6928a514f530b74fc3ab2636/nemo/collections/llm/api.py#L511-L521

# Failed -> ValueError: Invalid overwrite format: llama32_1b
nemo llm import llama3_8b source="hf://meta-llama/Llama-3.1-8B"

# Succeeded
nemo llm import model=llama3_8b source="hf://meta-llama/Llama-3.1-8B"

This PR adds support for positional args by trying to retrieve the argument names first in LazyEntrypoint._add_overwrite.
A corresponding test has been added to verify these changes.