Add input_mappings and output_mappings attributes

[gabrielmbmb]

Description

This PR adds two new arguments:

• input_mappings argument to all the kind of steps allowing to map columns/keys from the data used as input for the step to the name of the inputs required by it.

  class GenerateResponse(Step):
      @property
      def inputs(self) -> List[str]:
          return ["instruction"]
  
      @property
      def outputs(self) -> List[str]:
          return ["response"]
  
      def process(self, inputs: StepInput) -> StepOutput:
          for input in inputs:
              input["response"] = "-------> " + input["instruction"]
          yield inputs
  
  step = GenerateResponse(name="generate_response", input_mappings={"instruction": "prompt})
  # The `process_applying_mappings` will replace all the keys `prompt` by `instruction`
  step.process_applying_mappings([{"prompt": "Is `distilabel` awesome?"}])

• output_mappings argument for the Step and GlobalStep allowing to rename the outputs of the step:

  class GenerateResponse(Step):
      @property
      def inputs(self) -> List[str]:
          return ["instruction"]
  
      @property
      def outputs(self) -> List[str]:
          return ["response"]
  
      def process(self, inputs: StepInput) -> StepOutput:
          for input in inputs:
              input["response"] = "-------> " + input["instruction"]
          yield inputs
  
  step = GenerateResponse(name="generate_response", output_mappings={"response": "generation})
  # The `process_applying_mappings` will replace all the output keys `response` by `generation`
  step.process_applying_mappings([{"instruction": "Is `distilabel` awesome?"}])

[alvarobartt]

Hi here! LGTM overall, just to take into account that the DAG validation will need to use the mappings if there, so maybe a get_inputs function needs to be implemented. Besides that I would rename outputs_mappings to output_mappings to be consistent with input_mappings and to not add the plural to both words which IMO is confusing. Additionally I would also like to see what would the user workflow be for a step that has a predefined set of inputs already.

[alvarobartt]

Additionally I would also like to see what would the user workflow be for a step that has a predefined set of inputs already.

Would this be an user workflow to define the correct {input,output}_mappings?

from ... import CustomTask

print(CustomTask.inputs)
# ["question", "options"]
print(CustomTask.outputs)
# ["answer"]

input_mappings = {"question": "Q", "options": "O"}
output_mappings = {"answer": "A"}

i.e. to be able to explore the defaults before applying the mappings? Also the mappings should be disabled when there are no inputs / outputs right? Are we also handling that?