spec: add backend sampling support for eagle3 (#24655)

Author: ruixiang63

common/speculative.cpp

@@ -418,6 +418,9 @@ struct common_speculative_impl_draft_eagle3 : public common_speculative_impl {
 
     std::vector<common_sampler_ptr> smpls;
 
+    // backend sampler chain per seq, attached to ctx_dft
+    std::vector<llama_sampler *> backend_chains;
+
     int32_t n_embd_dec = 0;       // draft hidden size
     int32_t n_embd_enc = 0;       // target_layer_ids_n * target_hidden_size
     int32_t n_embd_tgt = 0;       // target model hidden size
@@ -443,7 +446,7 @@ struct common_speculative_impl_draft_eagle3 : public common_speculative_impl {
         , params(params.draft)
     {
         LOG_INF("%s: adding speculative implementation 'draft-eagle3'\n", __func__);
-        LOG_INF("%s: - n_max=%d, n_min=%d, p_min=%f\n", __func__, params.draft.n_max, params.draft.n_min, params.draft.p_min);
+        LOG_INF("%s: - n_max=%d, n_min=%d, p_min=%f, backend_sampling=%d\n", __func__, params.draft.n_max, params.draft.n_min, params.draft.p_min, (int) params.draft.backend_sampling);
 
         auto * ctx_tgt = this->params.ctx_tgt;
         auto * ctx_dft = this->params.ctx_dft;
@@ -478,6 +481,22 @@ struct common_speculative_impl_draft_eagle3 : public common_speculative_impl {
             s.reset(common_sampler_init(llama_get_model(ctx_dft), sparams));
         }
 
+        // offload draft sampling to the backend
+        backend_chains.assign(n_seq, nullptr);
+        if (this->params.backend_sampling) {
+            for (llama_seq_id seq_id = 0; seq_id < (llama_seq_id) n_seq; ++seq_id) {
+                llama_sampler * chain = llama_sampler_chain_init(llama_sampler_chain_default_params());
+                llama_sampler_chain_add(chain, llama_sampler_init_top_k(10));
+
+                if (!llama_set_sampler(ctx_dft, seq_id, chain)) {
+                    LOG_WRN("%s: backend offload failed for seq_id=%d; using CPU sampler\n", __func__, (int) seq_id);
+                    llama_sampler_free(chain);
+                    chain = nullptr;
+                }
+                backend_chains[seq_id] = chain;
+            }
+        }
+
         // turn on extraction of the target layers' input embeddings
         for (uint32_t k = 0; k < target_layer_ids_n; ++k) {
             llama_set_embeddings_layer_inp(ctx_tgt, (uint32_t) target_layer_ids[k], true);
@@ -496,6 +515,18 @@ struct common_speculative_impl_draft_eagle3 : public common_speculative_impl {
     }
 
     ~common_speculative_impl_draft_eagle3() override {
+        auto * ctx_dft = this->params.ctx_dft;
+        for (llama_seq_id seq_id = 0; seq_id < (llama_seq_id) backend_chains.size(); ++seq_id) {
+            if (backend_chains[seq_id] == nullptr) {
+                continue;
+            }
+            if (ctx_dft) {
+                llama_set_sampler(ctx_dft, seq_id, nullptr);
+            }
+            llama_sampler_free(backend_chains[seq_id]);
+        }
+        backend_chains.clear();
+
         if (batch.token != nullptr) {
             free(batch.token);
             batch.token = nullptr;