code: def _model_num_inputs(model):
      """Returns the number of input variables"""
      graph = model.graph
      if len(graph.input) != 1:
          raise ValueError(
              f"Model graph input field is multi-valued {graph.input}. A single value is required."
          )
      return _tensor_size(graph.input[0])
  
file_path: OMLT/src/omlt/gbt/model.py
label_desc: Use image node_id 1 for calling a global function with example usage: _model_num_inputs(model) and returns: _tensor_size