In a SeldonDeployment resource definition you have a graph section. The top-level element is the first step and it contains children for a next step. Here is a snippet from an example showing a transformer being used to transform a request before passing it on to a model:

    graph:
      children:
        - children: []
          implementation: XGBOOST_SERVER
          name: model
          modelUri: s3://model-image
          type: MODEL
      endpoint:
        type: REST
      name: feature-transformer
      type: TRANSFORMER

There's an example in the seldon documentation where steps such as clean and tokenize are used to perform inference for natural language processing. That example has multiple steps as type MODEL. There's also discussion on this in the seldon slack channel.