Use parameter-parameter and metadata-parameter edges

Edges connect the source and target nodes to each other in pipelines. They are often defined by setting the output of the source node to be the input of the target node. 

In addition, it is possible to define the pipeline edges with parameters. This means that the chosen parameter from the source node will be passed to a parameter in the target node. The passed value will be used during the execution of the target node instead of the default value from valohai.yaml.

Similarly, it is possible to collect metadata in the source node and pass it into a parameter in the target node.

Note that in both cases, the parameter where the values are passed has to be defined in the yaml for the target node. 

Example:

- step:
    name: source-node
    image: tensorflow/tensorflow:2.5.0
    command:
    - pip install valohai-utils 
    - python mycode.py {parameters}
    parameters:
    - name: param1
      default: one
      description: This will be passed to target-node
      type: string
- step:
    name: target-node
    image: tensorflow/tensorflow:2.5.0
    command:
    - pip install valohai-utils 
    - python myothercode.py {parameters}
    parameters:
    - name: param1
      default: one
      description: This will retain its value
      type: string
    - name: param2
      default: two
      description: In the pipeline, the value will be replace by param1 value from source-node
      type: string
    - name: param3
      default: three
      description: In the pipeline, the value will be replace by mymetadata value from source-node
      type: string

- pipeline:
    name: Parameter edges
    edges:
    - configuration: {}
      source: source-node.parameter.param1
      target: target-node.parameter.param2
    - configuration: {}
      source: source-node.metadata.mymetadata
      target: target-node.parameter.param3 
    nodes:
    - name: source-node
      override: {}
      step: source-node
      type: execution
    - name: target-node
      override: {}
      step: target-node
      type: execution