Removes the code path of MultiMessage from pre-process stages, post-process stages, inference stages, etc.

[yczhang-nv]

Previously the stages support processing both ControlMessage and MultiMessage - need to remove the code paths for MultiMessage from the following stages (from both C++ & Python implementation):

• Pre-process stages:

  • deserialize_stage
  • preprocess_base_stage
  • preprocess_ae_stage
  • preprocess_fil_stage
  • preprocess_nlp_stage
  • train_ae_stage

• Post-process stages:

  • serialize_stage
  • add_scores_stage_base_stage
  • add_scores_stage
  • add_classification_stage
  • filter_detections_stage
  • generate_viz_frames_stage
  • ml_flow_drift_stage
  • time_series_stage
  • validation_stage

• Inference stages:

  • auto_encoder_inference_stage
  • identity_inference_stage
  • inference_stage
  • pytorch_inference_stage
  • triton_inference_stage

• Output stages:

  • write_to_vector_db_stage

The related unit tests should also updated to use only ControlMessage.