common : skip model validation when --help is requested (#17755)
This commit skips the model validation check when the user specifies the
--help option.
The motivation for this is that currently and error is thrown before the
--help could be processed. Now skips validation if params.usage is set,
allowing help to display without requiring --model.
Resolves: https://github.com/ggml-org/llama.cpp/issues/17754