llama : refactor sampling_info to use buffer_view template (#19368)

* llama : refactor sampling_info to use buffer_view template

This commit updates the sampling_info struct in llama-context to use a
buffer_view template for the logits, probs, sampled tokens, and
candidates buffers.

The motivation for this is to simplify the code, improve type safety
and readability.

src/llama-impl.h

@@ -49,6 +49,16 @@ struct time_meas {
     int64_t & t_acc;
 };
 
+template <typename T>
+struct buffer_view {
+    T * data;
+    size_t size = 0;
+
+    bool has_data() const {
+        return data && size > 0;
+    }
+};
+
 void replace_all(std::string & s, const std::string & search, const std::string & replace);
 
 // TODO: rename to llama_format ?