common : allow --offline in llama download (#25091)
Expose the existing --offline flag to `llama download` so a script can
run it to check whether a model is already cached and ready to be served
without touching the network.
Also fix a latent use-after-free in the URL-task on_done callback:
first_path is block-scoped and was captured by reference, but invoked
after the block ends.
Signed-off-by: Adrien Gallouët <angt@huggingface.co>
