spec: enable backend sampling for both dflash & dspark (#26958)

* dflash: enable backend sampling for both dflash & dspark

* enable p_min > 0 in backend sampling and add guard

* cont : add TODO

---------

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

common/speculative.cpp

@@ -912,6 +912,9 @@ struct common_speculative_impl_draft_dflash : 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
@@ -985,6 +988,22 @@ struct common_speculative_impl_draft_dflash : public common_speculative_impl {
             s.reset(common_sampler_init(model_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)) {
+                    SPC_WRN("backend offload failed for seq_id=%d; using CPU sampler\n", (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);
@@ -995,6 +1014,18 @@ struct common_speculative_impl_draft_dflash : public common_speculative_impl {
     }
 
     ~common_speculative_impl_draft_dflash() 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();
+
         llama_batch_free(batch);
         llama_batch_free(batch_inject);
     }
@@ -2263,6 +2294,23 @@ common_params common_base_params_to_speculative(const common_params & params) {
     result.n_outputs_max = params.n_parallel;
     result.n_outputs_max_per_seq = 1;
 
+    // dflash/dspark decode the whole noise block in a single pass and sample every block position on the backend
+    // TODO: refactor such properties to be announced by the speculative types
+    //       something like `struct common_speculative_type_props common_speculative_type_get_props(...);`
+    const bool has_block_draft = std::any_of(
+        params.speculative.types.begin(), params.speculative.types.end(),
+        [](common_speculative_type t) {
+            return t == COMMON_SPECULATIVE_TYPE_DRAFT_DFLASH || t == COMMON_SPECULATIVE_TYPE_DRAFT_DSPARK;
+        });
+    if (has_block_draft) {
+        // per-seq output positions: DFlash decodes anchor + n_max masks (n_max + 1); DSpark n_max -> +1 covers both
+        const int32_t per_seq = std::max(1, params_spec.n_max + 1);
+        result.n_outputs_max = params.n_parallel * per_seq;
+        if (params_spec.backend_sampling) {
+            result.n_outputs_max_per_seq = per_seq;
+        }
+    }
+
     return result;
 }