llama : add llama_model_ftype_name() (#25134)

* llama : add llama_model_ftype_name()

Expose the model file type (quantization) name, e.g. "Q8_0" or
"Q4_K - Medium", through a new public C API. The returned pointer is
valid for the lifetime of the model and nullptr when the model is
invalid or the file type is unknown.

Signed-off-by: Adrien Gallouët <angt@huggingface.co>

* Export enum

Signed-off-by: Adrien Gallouët <angt@huggingface.co>

* s/llama_model_ftype_name/llama_ftype_name/

Signed-off-by: Adrien Gallouët <angt@huggingface.co>

* Move "(guessed)" to the front in llama_ftype_name

Prepend the "(guessed)" label instead of appending it. This allows removing
the non-thread-safe static std::string, making the function allocation-free.

Signed-off-by: Adrien Gallouët <angt@huggingface.co>

* Add LLAMA_FTYPE_PREFIX

Signed-off-by: Adrien Gallouët <angt@huggingface.co>

* Dont check for model

Signed-off-by: Adrien Gallouët <angt@huggingface.co>

---------

Signed-off-by: Adrien Gallouët <angt@huggingface.co>

src/llama-model.h

@@ -637,6 +637,8 @@ struct llama_model {
 
     std::string desc() const;
 
+    llama_ftype ftype() const;
+
     size_t size() const; // file size
     size_t n_tensors() const;
     size_t n_devices() const;