Fix off-by-one errors in generation code and token streaming callback.

In the generation code we were feeding the last token of the prompt
twice through the transformer. The new version fixes that and also
works in the case where Prefill is completely disabled.

gemma.cc

@@ -666,12 +666,16 @@ void GenerateImpl(GemmaImpl<TConfig>& gemma, size_t max_tokens,
 
   size_t pos_gen_start = pos_offset;
   int token = prompt.at(pos_offset);
+  stream_token(token, 0);
   for (size_t generate_pos = 0;
        pos < max_tokens && generate_pos < max_generated_tokens;
        ++pos, ++pos_offset, ++generate_pos) {
     Transformer(token, pos, c_weights, activations, kv_cache, pool, inner_pool);
     float* final_activation = activations.x.data();
-    if (pos_offset >= prompt_size) {
+    // The condition below is always true if we are doing Prefill above.
+    // We keep it here for clarity so that the code is correct even if Prefill
+    // is disabled.
+    if (pos_offset >= prompt_size - 1) {
       PROFILER_ZONE("Gen.Embedding");
       // Generation phase
       MatVec<kVocabSize, TConfig::kModelDim>(
@@ -681,10 +685,15 @@ void GenerateImpl(GemmaImpl<TConfig>& gemma, size_t max_tokens,
       Softmax(activations.logits.data(), kVocabSize);
       token = SampleTopK<TConfig::kTopK>(activations.logits.data(), kVocabSize,
                                          gen, temperature, accept_token);
-    }
       if (!stream_token(token, activations.logits[token])) {
         token = EOS_ID;
       }
+    } else {
+      // We would take this branch if we were not doing Prefill but would
+      // process the tokens of the prompt one at a time.
+      token = prompt.at(pos_offset + 1);
+      stream_token(token, 0);
+    }
     if (token == EOS_ID) {
       if (verbosity >= 2) {
         const double gen_end = hwy::platform::Now();

run.cc

@@ -116,7 +116,8 @@ void ReplGemma(gcpp::Gemma& model, gcpp::KVCache& kv_cache,
                        verbosity](int token, float) {
     ++abs_pos;
     ++current_pos;
-    if (current_pos < prompt_size) {
+    // <= since position is incremented before
+    if (current_pos <= prompt_size) {
       std::cerr << "." << std::flush;
     } else if (token == gcpp::EOS_ID) {
       if (!args.multiturn) {