spec : add probabilistic sampling for simple draft and MTP (#27694)

* Make the drafter probabilistic and the target verify by rejection sampling

* Drop stale spec_draft_q before drafting

* Fallback to argmax sampling for grammar-constrained requests and adding flag for enabling probabilistic draft sampling. Default flag value is greedy.

* Support grammar-constrained requests in rejection sampling

* Fix - renormalize distribution after masking

* copy rng on sampler copy and re-accept drafted tokens on replay

* Fix draft sampler sharing the target's rng stream

* Simplify the rejection sampler's inputs and move replay to the server

* Truncate the draft candidates along with the draft

---------

Co-authored-by: praneshgo <227579474+praneshgo@users.noreply.github.com>
Co-authored-by: Pranesh Gonegandla <pgonegandla@nvidia.com>

common/sampling.cpp

@@ -12,6 +12,7 @@
 #include <climits>
 #include <cmath>
 #include <cstring>
+#include <random>
 #include <unordered_map>
 #include <vector>
 
@@ -121,6 +122,9 @@ struct common_sampler {
 
     llama_token_data_array cur_p;
 
+    // for rejection sampling; independent of the draft, or the target distribution is not preserved
+    std::mt19937 rng;
+
     void reset() {
         prev.clear();
 
@@ -432,6 +436,8 @@ struct common_sampler * common_sampler_init(
         /* .prev    = */ ring_buffer<llama_token>(std::max(32, params.n_prev)),
         /* .cur     = */ {},
         /* .cur_p   = */ {},
+        // mix it, the chain and the draft are seeded from this one too
+        /* .rng     = */ std::mt19937(llama_sampler_get_seed(chain) ^ 0x9e3779b9u),
     };
 
     return result;
@@ -515,6 +521,7 @@ struct common_sampler * common_sampler_clone(common_sampler * gsmpl) {
         /* .prev    = */ gsmpl->prev,
         /* .cur     = */ gsmpl->cur,
         /* .cur_p   = */ gsmpl->cur_p,
+        /* .rng     = */ gsmpl->rng,
     };
 }
 
@@ -535,6 +542,7 @@ void common_sampler_copy(const common_sampler * src, common_sampler * dst) {
     dst->cur        = src->cur;
     dst->cur_p      = src->cur_p;
     dst->cur_p.data = src->cur_p.data ? dst->cur.data() : nullptr; // re-point to dst's buffer
+    dst->rng        = src->rng;
     dst->t_total_us = src->t_total_us;
 }
 
@@ -709,6 +717,124 @@ std::vector<llama_token> common_sampler_sample_and_accept_n(struct common_sample
     return result;
 }
 
+static float prob_of(const llama_token_data * data, size_t n, llama_token id) {
+    for (size_t k = 0; k < n; ++k) {
+        if (data[k].id == id) {
+            return data[k].p;
+        }
+    }
+    return 0.0f;
+}
+
+// Accept a drafted token with probability min(1, p/q), else draw from norm(max(0, p - q)).
+// Preserves the target distribution exactly, and accepts more often than matching does when the
+// draft samples instead of taking its argmax.
+std::vector<llama_token> common_sampler_sample_and_accept_n_rejection(struct common_sampler * gsmpl, struct llama_context * ctx, const std::vector<int> & idxs, const llama_tokens & draft, const std::vector<std::vector<llama_token_data>> & draft_q, bool grammar_first) {
+    GGML_ASSERT(idxs.size()    == draft.size() + 1 && "idxs.size() must be draft.size() + 1");
+    GGML_ASSERT(draft_q.size() == draft.size() && "draft_q must have one entry per draft token");
+
+    std::vector<llama_token> result;
+    result.reserve(idxs.size());
+
+    // draws come from the sampler's own stream, so they stay independent of what was drafted
+    std::uniform_real_distribution<float> uni(0.0f, 1.0f);
+
+    std::vector<llama_token_data> residual;
+
+    std::vector<llama_token_data> cand; // candidate array masked by the grammar, if there is one
+
+    size_t i = 0;
+    for (; i < draft.size(); i++) {
+        // leaves the target distribution in the candidate array
+        const llama_token id_tgt = common_sampler_sample(gsmpl, ctx, idxs[i], grammar_first);
+
+        const auto * cur_p = common_sampler_get_candidates(gsmpl, true);
+        const auto & q     = draft_q[i];
+
+        const bool masked = !grammar_first && grammar_should_apply(gsmpl);
+        if (masked) {
+            cand.assign(cur_p->data, cur_p->data + cur_p->size);
+            llama_token_data_array arr = { cand.data(), cand.size(), -1, false };
+            llama_sampler_apply(gsmpl->grmr, &arr);
+        }
+
+        // a candidate the grammar rejects carries no probability, whatever the target thinks
+        auto p_raw = [&](size_t k) {
+            return masked && cand[k].logit == -INFINITY ? 0.0f : cur_p->data[k].p;
+        };
+
+        // masking drops probability mass, so rescale what is left or the residual is over-weighted
+        float p_sum = 0.0f;
+        if (masked) {
+            for (size_t k = 0; k < cur_p->size; ++k) {
+                p_sum += p_raw(k);
+            }
+        }
+
+        const float p_norm = masked && p_sum > 0.0f ? 1.0f/p_sum : 1.0f;
+
+        auto p_of = [&](size_t k) {
+            return p_raw(k)*p_norm;
+        };
+
+        // q_x is never 0 for a token the draft produced, but guard the divide
+        const float q_x = prob_of(q.data(), q.size(), draft[i]);
+
+        float p_x = 0.0f;
+        for (size_t k = 0; k < cur_p->size; ++k) {
+            if (cur_p->data[k].id == draft[i]) {
+                p_x = p_of(k);
+                break;
+            }
+        }
+
+        if (q_x > 0.0f && (p_x >= q_x || uni(gsmpl->rng) < p_x / q_x)) {
+            common_sampler_accept(gsmpl, draft[i], true);
+            result.push_back(draft[i]);
+            continue;
+        }
+
+        // rejected: tokens outside q's support keep all of p
+        residual.clear();
+        float sum = 0.0f;
+        for (size_t k = 0; k < cur_p->size; ++k) {
+            const float r = p_of(k) - prob_of(q.data(), q.size(), cur_p->data[k].id);
+            if (r > 0.0f) {
+                residual.push_back({ cur_p->data[k].id, 0.0f, r });
+                sum += r;
+            }
+        }
+
+        llama_token id = id_tgt;
+        if (sum > 0.0f) {
+            float u = uni(gsmpl->rng) * sum;
+            id = residual.back().id;
+            for (const auto & e : residual) {
+                u -= e.p;
+                if (u <= 0.0f) {
+                    id = e.id;
+                    break;
+                }
+            }
+        }
+
+        common_sampler_accept(gsmpl, id, true);
+        result.push_back(id);
+
+        break;
+    }
+
+    if (i == draft.size()) {
+        const llama_token id = common_sampler_sample(gsmpl, ctx, idxs[i], grammar_first);
+
+        common_sampler_accept(gsmpl, id, true);
+
+        result.push_back(id);
+    }
+
+    return result;
+}
+
 std::vector<llama_token> common_sampler_sample_and_accept_n(struct common_sampler * gsmpl, struct llama_context * ctx, const llama_tokens & draft, bool grammar_first) {
     std::vector<int> idxs(draft.size() + 1);
     for (size_t i = 0; i < idxs.size(); ++i) {