Factor out DecodeStepT from GenerateT into a separate function.

This will be useful for adding sampling functionality like beam decoding, parallel sampling, cot decoding (as described in the [Chain-of-Thought Reasoning Without Prompting paper](https://arxiv.org/abs/2402.10200))

PiperOrigin-RevId: 725066435

gemma/gemma-inl.h

@@ -27,6 +27,7 @@
 #include "gemma/common.h"
 #include "gemma/configs.h"
 #include "gemma/gemma.h"
+#include "gemma/kv_cache.h"
 #include "gemma/weights.h"
 #include "paligemma/image.h"
 #include "util/allocator.h"
@@ -1217,6 +1218,60 @@ HWY_INLINE SampleFunc ChooseSampleFunc(const RuntimeConfig& runtime_config) {
   };
 }
 
+template <typename T>
+void DecodeStepT(const ModelWeightsPtrs<T>& weights,
+                const RuntimeConfig& runtime_config,
+               const QueriesPromptTokens& queries_prompt,
+               const size_t query_idx_start,
+               const KVCaches& kv_caches,
+               const QueriesPos& queries_prefix_end,
+               const hwy::Divisor div_seq_len,
+               const size_t vocab_size,
+               const SampleFunc& sample_token,
+               double prefill_start,
+               double gen_start,
+               Activations& activations,
+               TokenStreamer& token_streamer,
+               std::vector<int>& gen_tokens,
+               TimingInfo& timing_info,
+               size_t max_generated_tokens,
+               const QueriesMutablePos& queries_mutable_pos,
+               bool& all_queries_eos) {
+  const size_t num_queries = queries_prompt.size();
+  // Decode generates one token per query and increments
+  // queries_mutable_pos.
+  Transformer(QueriesToken(gen_tokens.data(), num_queries),
+              queries_mutable_pos, queries_prefix_end, weights, activations,
+              div_seq_len, kv_caches, runtime_config.layers_output,
+              runtime_config.activations_observer);
+  // queries_pos are incremented by Transformer.
+
+  all_queries_eos = true;
+  {
+    PROFILER_ZONE("Gen.EmbeddingMatmul");
+    // Compute logits from last layer activations.
+    MatMul(ConstMatFromBatch(num_queries, activations.x),
+            ConstMatFromWeights(weights.embedder_input_embedding),
+            /*add=*/nullptr, *activations.env,
+            RowPtrFromBatch(activations.logits));
+  }
+  PROFILER_ZONE("Gen.Softcap+Sample+Stream");
+  for (size_t query_idx = 0; query_idx < num_queries; ++query_idx) {
+    float* HWY_RESTRICT logits = activations.logits.Batch(query_idx);
+    MaybeLogitsSoftCap(weights.weights_config.final_cap, logits,
+                        vocab_size);
+    const TokenAndProb tp = sample_token(logits, vocab_size);
+    timing_info.NotifyGenerated(prefill_start, gen_start);
+
+    const bool is_eos =
+        token_streamer(query_idx_start + query_idx,
+                        queries_mutable_pos[query_idx], tp.token, tp.prob);
+    all_queries_eos &= is_eos;
+    gen_tokens[query_idx] = is_eos ? runtime_config.eos_id : tp.token;
+  }
+}
+
+
 // Generates one continuation for each query in `queries_prompt`, which is one
 // qbatch whose size is at most the `batch_size` passed to
 // `activations.Allocate`.
@@ -1310,37 +1365,12 @@ void GenerateT(const ModelWeightsStorage& model, Activations& activations,
     const size_t vocab_size = model.Config().vocab_size;
     const double gen_start = hwy::platform::Now();
     for (size_t gen = 0; gen < max_generated_tokens; ++gen) {
-      // Decode generates one token per query and increments
-      // queries_mutable_pos.
-      Transformer(QueriesToken(gen_tokens.data(), num_queries),
-                  queries_mutable_pos, queries_prefix_end, weights, activations,
-                  div_seq_len, kv_caches, runtime_config.layers_output,
-                  runtime_config.activations_observer);
-      // queries_pos are incremented by Transformer.
-
       bool all_queries_eos = true;
-      {
-        PROFILER_ZONE("Gen.EmbeddingMatmul");
-        // Compute logits from last layer activations.
-        MatMul(ConstMatFromBatch(num_queries, activations.x),
-               ConstMatFromWeights(weights.embedder_input_embedding),
-               /*add=*/nullptr, *activations.env,
-               RowPtrFromBatch(activations.logits));
-      }
-      PROFILER_ZONE("Gen.Softcap+Sample+Stream");
-      for (size_t query_idx = 0; query_idx < num_queries; ++query_idx) {
-        float* HWY_RESTRICT logits = activations.logits.Batch(query_idx);
-        MaybeLogitsSoftCap(weights.weights_config.final_cap, logits,
-                           vocab_size);
-        const TokenAndProb tp = sample_token(logits, vocab_size);
-        timing_info.NotifyGenerated(prefill_start, gen_start);
-
-        const bool is_eos =
-            token_streamer(query_idx_start + query_idx,
-                           queries_mutable_pos[query_idx], tp.token, tp.prob);
-        all_queries_eos &= is_eos;
-        gen_tokens[query_idx] = is_eos ? runtime_config.eos_id : tp.token;
-      }
+      DecodeStepT<T>(weights, runtime_config, queries_prompt, query_idx_start,
+                  kv_caches, queries_prefix_end, div_seq_len, vocab_size,
+                  sample_token, prefill_start, gen_start, activations,
+                  token_streamer, gen_tokens, timing_info, max_generated_tokens,
+                  queries_mutable_pos, all_queries_eos);
       if (all_queries_eos) break;
     }  // foreach token to generate
     timing_info.NotifyGenerateDone(gen_start);