Add step decorator

[gabrielmbmb]

Description

This PR adds the step decorator which allows to create an Step, GlobalStep or GeneratorStep defining its process function. It will allow creating simpler steps more easily, but for more complex steps the recommended way is to use the inheritance method.

Closes #374

# Normal step
@step(inputs=["instruction"], outputs=["generation"])
def GenerationStep(inputs: StepInput, dummy_generation: RuntimeParameter[str]) -> StepOutput:
    for input in inputs:
        input["generation"] = dummy_generation
    yield inputs

# Global step
@step(inputs=["instruction"], step_type="global")
def FilteringStep(inputs: StepInput, max_length: RuntimeParameter[int] = 256) -> StepOutput:
    yield [
        input
        for input in inputs
        if len(input["instruction"]) <= max_length
    ]

# Generator step
@step(outputs=["num"], step_type="generator")
def RowGenerator(num_rows: RuntimeParameter[int] = 500) -> GeneratorStepOutput:
    data = list(range(num_rows))
    for i in range(0, len(data), 100):
        last_batch = i + 100 >= len(data)
        yield [{"num": num} for num in data[i : i + 100]], last_batch