Recompute KV cache for Phi3 when switching from short to long factor (#…

…1161)

Recompute KV cache for Phi3 when switching from short to long factor.

Verified that this PR fixes the issue for:
1. Phi3.5 mini
2. Phi3 mini 128K
3. Phi3 small
4. Phi3 medium

src/generators.cpp

@@ -389,6 +389,20 @@ void Generator::GenerateNextToken() {
   ThrowErrorIfSessionTerminated(state_->session_terminated_);
   if (search_->GetSequenceLength() == 0 && !computed_logits_)
     throw std::runtime_error("GenerateNextToken called with no prior state. Please call AppendTokens, SetLogits, or params.SetInputs before calling GenerateNextToken.");
+
+  // TODO: Extend the solution to make it work for batch size > 1, num beams > 1, multimodal and DML
+  // Phi3 model switches from short factor to long factor at 4097 (original_max_position_embeddings+1) token, needs Recomputation of Position IDs and KV Cache
+  // at this stage which is achieved by rewinding to zero and appending the current sequence
+  // Scenarios where this solution works: Batch size = 1, Num beams = 1, decoder model, EP is either CPU or CUDA
+  // Scenarios where it doesn't work: Batch size > 1 OR Num beams > 1 OR Multimodal model (like phi3 vision) OR EP is DML
+  if (search_->params_->BatchBeamSize() == 1) {
+    if (((search_->GetSequenceLength() == 4097) && (model_->config_->model.type == "phi3" || model_->config_->model.type == "phimoe")) || ((search_->GetSequenceLength() == 8197) && (model_->config_->model.type == "phi3small"))) {
+      auto current_seq = cpu_span<int32_t>(GetSequence(0).CopyDeviceToCpu());
+      RewindToLength(0);
+      AppendTokens(current_seq);
+    }
+  }
+
   if (!computed_logits_) {
     auto next_tokens = search_->GetNextTokens();
     if (last_action_ == Action::rewound)