Add constructor fromPtr() for all flutter class with factory ctor. (#1667)

Co-authored-by: wangrui <wangrui@oecore.com>

flutter/sherpa_onnx/lib/src/offline_speaker_diarization.dart

@@ -94,6 +94,9 @@ class OfflineSpeakerDiarizationConfig {
 }
 
 class OfflineSpeakerDiarization {
+  OfflineSpeakerDiarization.fromPtr(
+      {required this.ptr, required this.config, required this.sampleRate});
+
   OfflineSpeakerDiarization._(
       {required this.ptr, required this.config, required this.sampleRate});