Add LODR support to online and offline recognizers (#2026)

This PR integrates LODR (Level-Ordered Deterministic Rescoring) support from Icefall into both online and offline recognizers, enabling LODR for LM shallow fusion and LM rescore.

- Extended OnlineLMConfig and OfflineLMConfig to include lodr_fst, lodr_scale, and lodr_backoff_id.
- Implemented LodrFst and LodrStateCost classes and wired them into RNN LM scoring in both online and offline code paths.
- Updated Python bindings, CLI entry points, examples, and CI test scripts to accept and exercise the new LODR options.

sherpa-onnx/csrc/lodr-fst.h

@@ -0,0 +1,75 @@
+// sherpa-onnx/csrc/lodr-fst.h
+//
+// Contains code copied from icefall/utils/ngram_lm.py
+// Copyright (c)  2023 Xiaomi Corporation
+//
+// Copyright (c)  2025 Tilde SIA (Askars Salimbajevs)
+
+
+#ifndef SHERPA_ONNX_CSRC_LODR_FST_H_
+#define SHERPA_ONNX_CSRC_LODR_FST_H_
+
+#include <memory>
+#include <string>
+#include <vector>
+#include <optional>
+#include <tuple>
+#include <unordered_map>
+#include <limits>
+#include <algorithm>
+#include <utility>
+
+#include "kaldifst/csrc/kaldi-fst-io.h"
+
+namespace sherpa_onnx {
+
+class Hypothesis;
+
+class LodrFst {
+ public:
+  explicit LodrFst(const std::string &fst_path, int32_t backoff_id = -1);
+
+  std::pair<std::vector<int32_t>, std::vector<float>> GetNextStateCosts(
+    int32_t state, int32_t label);
+
+  float GetFinalCost(int32_t state);
+
+  void ComputeScore(float scale, Hypothesis *hyp, int32_t offset);
+
+ private:
+  fst::StdVectorFst YsToFst(const std::vector<int64_t> &ys, int32_t offset);
+
+  std::vector<std::tuple<int32_t, float>> ProcessBackoffArcs(
+    int32_t state, float cost);
+
+  std::optional<std::tuple<int32_t, float>> GetNextStatesCostsNoBackoff(
+    int32_t state, int32_t label);
+
+  int32_t FindBackoffId();
+
+
+  int32_t backoff_id_ = -1;
+  std::unique_ptr<fst::StdConstFst> fst_;  // owned by this class
+};
+
+class LodrStateCost {
+ public:
+  explicit LodrStateCost(
+    LodrFst* fst,
+    const std::unordered_map<int32_t, float> &state_cost = {});
+
+    LodrStateCost ForwardOneStep(int32_t label);
+
+  float Score() const;
+  float FinalScore() const;
+
+ private:
+  // The fst_ is not owned by this class and borrowed from the caller
+  // (e.g. OnlineRnnLM).
+  LodrFst* fst_;
+  std::unordered_map<int32_t, float> state_cost_;
+};
+
+}  // namespace sherpa_onnx
+
+#endif  // SHERPA_ONNX_CSRC_LODR_FST_H_