Add support for naming tasks in @requires

[IanCal]

Description

This change lets a user name the tasks in the requires method like so:

@requires(daily=DailySummaryTask, weekly=WeeklySummaryTask)
class SummaryTask(Task):
    def run(self):
        with self.input()["daily"].open() as daily_results:
            ...

Motivation and Context

When we use the requires decorator, we can save a lot of time and hassle passing parameters around. One issue we have hit is that the run method only sees a list of targets, with no context as to which earlier task generated the results.

Luigi already supports named inputs, by returning a dictionary from the requires function. This change just allows passing in kwargs to the decorator and creates a requires method using those. I added this to the inherits decorator as well,

There wasn't a sensible option in my mind if you try and pass in both, so I made it throw an error in that case.

This doesn't include an update to the documentation but I can add some docs if the approach is OK.

This doesn't handle any fancier nested versions, just a simple dictionary.

Have you tested this? If so, how?

I added some basic unit tests, but can expand these, and have put an example program below.

It's largely taken from demo code we're working on, and is I hope a relatively simple change.

Example:

from luigi.util import requires
from luigi import Task, Parameter, build
from luigi.local_target import LocalTarget


class DependencyA(Task):
    def run(self):
        with self.output().open("w") as f_out:
            f_out.write("DependencyA file output")

    def output(self):
        return LocalTarget(f"output/{self.get_task_family()}")


class DependencyB(Task):
    def run(self):
        with self.output().open("w") as f_out:
            f_out.write("DependencyB file output")

    def output(self):
        return LocalTarget(f"output/{self.get_task_family()}")


@requires(dep_a=DependencyA, dep_b=DependencyB)
class ExampleTask(Task):
    def run(self):
        dep_a_data = self.input()["dep_a"].open().read()
        dep_b_data = self.input()["dep_b"].open().read()
        print("Data", dep_a_data, dep_b_data)

build([
    ExampleTask()
], local_scheduler=True)

[IanCal]

This error isn't required as inherits will be called, and inherits throws the same errors.