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

1 files changed, 12 insertions, 0 deletions

diff --git a/llama.h b/llama.h
index 686463a..4596b1e 100644
--- a/llama.h
+++ b/llama.h
@@ -309,6 +309,18 @@ extern "C" {
     /// @details Frequency and presence penalties described in OpenAI API https://platform.openai.com/docs/api-reference/parameter-details.
     LLAMA_API void llama_sample_frequency_and_presence_penalties(struct llama_context * ctx, llama_token_data_array * candidates, const llama_token * last_tokens, size_t last_tokens_size, float alpha_frequency, float alpha_presence);
 
+    /// @details Apply classifier-free guidance to the logits as described in academic paper "Stay on topic with Classifier-Free Guidance" https://arxiv.org/abs/2306.17806
+    /// @param candidates A vector of `llama_token_data` containing the candidate tokens, the logits must be directly extracted from the original generation context without being sorted.
+    /// @params guidance_ctx A separate context from the same model. Other than a negative prompt at the beginning, it should have all generated and user input tokens copied from the main context.
+    /// @params scale Guidance strength. 1.0f means no guidance. Higher values mean stronger guidance.
+    /// @params smooth_factor Smooth factor between guidance logits and original logits. 1.0f means only use guidance logits. 0.0f means only original logits.
+    LLAMA_API 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);
+
     /// @details Sorts candidate tokens by their logits in descending order and calculate probabilities based on logits.
     LLAMA_API void llama_sample_softmax(struct llama_context * ctx, llama_token_data_array * candidates);