sampling : add debug log when backend sampler selects token

This commit adds a debug log statement in the llama_sampler_sample
to indicate when a backend sampler has selected a token for a given
index.

The modification helps in tracing the sampling process and understanding
the flow of control when backend samplers are used.

src/llama-sampling.cpp

@@ -446,6 +446,7 @@ llama_token llama_sampler_sample(struct llama_sampler * smpl, struct llama_conte
 
     // If a backend sampler has already sampled a token, return it.
     if (sampled_token != LLAMA_TOKEN_NULL) {
+        LLAMA_LOG_DEBUG("%s: Backend sampler selected token for idx %d. Skipping CPU samplers\n", __func__, idx);
         return sampled_token;
     }