backend sampling: support returning post-sampling probs (#22622)

* server: Never return 0.0 post-sampling probabilities

* backend sampling: support returning post-sampling probs

tools/server/server-context.cpp

@@ -1317,7 +1317,7 @@ private:
                 return false;
             }
 
-            const bool need_logits = task.params.sampling.n_probs > 0;
+            const bool need_pre_sample_logits = task.params.sampling.n_probs > 0 && !task.params.post_sampling_probs;
 
             bool backend_sampling = true;
 
@@ -1326,8 +1326,8 @@ private:
             // TODO: speculative decoding requires multiple samples per batch - not supported yet
             backend_sampling &= !(slot.can_speculate() && common_speculative_n_max(slot.spec.get(), task.params.speculative) > 0);
 
-            // TODO: getting post/pre sampling logits is not yet supported with backend sampling
-            backend_sampling &= !need_logits;
+            // TODO: getting pre sampling logits is not yet supported with backend sampling
+            backend_sampling &= !need_pre_sample_logits;
 
             // TODO: tmp until backend sampling is fully implemented
             if (backend_sampling) {
@@ -1504,6 +1504,12 @@ private:
             // set probability for top n_probs tokens
             result.probs.reserve(n_probs);
             for (size_t i = 0; i < n_probs; i++) {
+                // Some samplers do return 0.0 probabilities, others don't.
+                // Filter 0.0 probailities, to ensure the behavior is consistent.
+                if (cur_p->data[i].p == 0.0) {
+                    break;
+                }
+
                 result.probs.push_back({
                     cur_p->data[i].id,
                     common_token_to_piece(ctx, cur_p->data[i].id, special),