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/offline-lm.h

@@ -10,12 +10,24 @@
 
 #include "onnxruntime_cxx_api.h"  // NOLINT
 #include "sherpa-onnx/csrc/hypothesis.h"
+#include "sherpa-onnx/csrc/lodr-fst.h"
 #include "sherpa-onnx/csrc/offline-lm-config.h"
 
 namespace sherpa_onnx {
 
 class OfflineLM {
  public:
+  explicit OfflineLM(const OfflineLMConfig &config) : config_(config) {
+    if (!config_.lodr_fst.empty()) {
+      try {
+        lodr_fst_ = std::make_unique<LodrFst>(LodrFst(config_.lodr_fst,
+                                                    config_.lodr_backoff_id));
+      } catch (const std::exception& e) {
+        throw std::runtime_error("Failed to load LODR FST from: " +
+                                  config_.lodr_fst + ". Error: " + e.what());
+      }
+    }
+  }
   virtual ~OfflineLM() = default;
 
   static std::unique_ptr<OfflineLM> Create(const OfflineLMConfig &config);
@@ -43,6 +55,11 @@ class OfflineLM {
   // @param hyps It is changed in-place.
   void ComputeLMScore(float scale, int32_t context_size,
                       std::vector<Hypotheses> *hyps);
+
+ private:
+  std::unique_ptr<LodrFst> lodr_fst_;
+  float lodr_scale_;
+  OfflineLMConfig config_;
 };
 
 }  // namespace sherpa_onnx