common : add specific warning for multiple -m options

[danbev]

This commit updates the deprecation warning for multiple model options with a specific message that does not mention comma-separated values as an option.

The motivation for this is that currently it can be confusing if multiple model options are specified with llama-server as this would display the following warning:

DEPRECATED: argument '--model' specified multiple times, use comma-separated values instead (only last value will be used)

But llama-server does not support a comma-separated models option.

With this change the user is informed that multiple models can be supported in router mode if using llama-server:

DEPRECATED: multiple --model options are not supported (only last value will be used). Use router mode for llama-server (see --help)

[ngxson]

Unless I missed something, we never support multiple -m, so using the word "deprecated" here make one thinks that it used to be, but now no longer supported

[danbev]

Unless I missed something, we never support multiple -m, so using the word "deprecated" here make one thinks that it used to be, but now no longer supported

Yeah good point, and I was not sure about using "deprecated" here but thought I'd perhaps it was supported at some point that I was not aware of. Let me know and I'll change this to something else, perhaps just Warning or WARNING.

[ngxson]

But to start, in which case people may specify multiple -m?

I personally believe that multiple -m has never been an expected behavior since the beginning of this project, so it makes more sense to just throw an error instead.

[danbev]

But to start, in which case people may specify multiple -m?

This was suggested to me by an AI actually (can't recall which model) which is why I looked into this as I did not think this was supported.

so it makes more sense to just throw an error instead.

I agree, and if that is alright to do that here it would be preferred.

[ngxson]

This was suggested to me by an AI actually (can't recall which model) which is why I looked into this as I did not think this was supported.

In such case, I think there is no need to change anything at all.

If something need to changed, I expect to throw an error on any args that are not supposed to have multiple values. We have plenty of them: -c, -n, --temp, --top-k, -ngl, ...

[danbev]

In such case, I think there is no need to change anything at all.

I actually think this is a reason to change this as it might be suggested to users, and it is somewhat confusing as it is right now.

But I agree that throwing an error for all the options that don't actually support being specified multiple times would be nicer. I'll add a commit with a suggestion for this:

For example, using multiple --model options would result in an error:

$ llama-completion -m models/Qwen2.5-0.5B-Instruct.gguf --model models/Qwen2.5-0.5B-Instruct.gguf
error: argument '--model' cannot be specified multiple times

But using multiple --file options would only result in a warning (like it currently does):

$ llama-completion -m models/Qwen2.5-0.5B-Instruct.gguf -f test_file.txt -f test_file.txt
DEPRECATED: argument '-f' specified multiple times, use comma-separated values instead (only last value will be used)
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