text: Title: Facing problem when loading BERTopic Online Model 
  Body: Hi @MaartenGr ,
  
  **I am facing the below issue when, I try to load an Online BERTopic model from my disk.
  
  Issue --> 'In order to use `.partial_fit`, the cluster model should have a `.partial_fit` function.'
  
  I tried to debug the code flow where I found that in BERTopic.load() we have a function call to _create_model_from_files() which loads the saved model. 
  The logic goes like this
  ```
  empty_dimensionality_model = BaseDimensionalityReduction()
      empty_cluster_model = BaseCluster()
  
      # Fit BERTopic without actually performing any clustering
      topic_model = BERTopic(
              embedding_model=embedding_model,
              umap_model=empty_dimensionality_model,
              hdbscan_model=empty_cluster_model,
              **params
      )
  ```
  Here, for Online Topic Models we use River.DBSTREAM as cluster model. But when we save the model using BERTopic.save() [in safetensors format] we are not storing the cluster model data.... Because of that, While loading back we face this issue..
  
  Is there any quick fix for this issue??**