llama : add classifier-free guidance (#2135)

* Initial implementation

* Remove debug print

* Restore signature of llama_init_from_gpt_params

* Free guidance context

* Make freeing of guidance_ctx conditional

* Make Classifier-Free Guidance a sampling function

* Correct typo. CFG already means context-free grammar.

* Record sampling time in llama_sample_classifier_free_guidance

* Shift all values by the max value before applying logsoftmax

* Fix styling based on review

llama.cpp

@@ -2167,6 +2167,62 @@ void llama_sample_frequency_and_presence_penalties(struct llama_context * ctx, l
     }
 }
 
+static void llama_log_softmax(float * array, size_t size) {
+    float max_l = *std::max_element(array, array + size);
+    float sum = 0.f;
+    for (size_t i = 0; i < size; ++i) {
+        float p = expf(array[i] - max_l);
+        sum += p;
+        array[i] = p;
+    }
+
+    for (size_t i = 0; i < size; ++i) {
+        array[i] = logf(array[i] / sum);
+    }
+}
+
+void llama_sample_classifier_free_guidance(
+          struct llama_context * ctx,
+        llama_token_data_array * candidates,
+          struct llama_context * guidance_ctx,
+                         float   scale,
+                         float   smooth_factor) {
+    int64_t t_start_sample_us = t_start_sample_us = ggml_time_us();
+
+    assert(ctx);
+    auto n_vocab = llama_n_vocab(ctx);
+    assert(n_vocab == (int)candidates->size);
+    assert(!candidates->sorted);
+
+    std::vector<float> logits_base;
+    logits_base.reserve(candidates->size);
+    for (size_t i = 0; i < candidates->size; ++i) {
+        logits_base.push_back(candidates->data[i].logit);
+    }
+    llama_log_softmax(logits_base.data(), candidates->size);
+
+    float* logits_guidance = llama_get_logits(guidance_ctx);
+    llama_log_softmax(logits_guidance, n_vocab);
+
+    for (int i = 0; i < n_vocab; ++i) {
+        float logit_guidance = logits_guidance[i];
+        float logit_base = logits_base[i];
+        logits_guidance[i] = scale * (logit_base - logit_guidance) + logit_guidance;
+    }
+
+    llama_log_softmax(logits_guidance, n_vocab);
+
+    for (int i = 0; i < n_vocab; ++i) {
+        float logit_base = logits_base[i];
+        float logit_guidance = logits_guidance[i];
+
+        candidates->data[i].logit = smooth_factor * logit_guidance + (1.f - smooth_factor) * logit_base;
+    }
+
+    if (ctx) {
+        ctx->t_sample_us += ggml_time_us() - t_start_sample_us;
+    }
+}
 
 llama_token llama_sample_token_mirostat(struct llama_context * ctx, llama_token_data_array * candidates, float tau, float eta, int m, float * mu) {
     assert(ctx);