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Outerbounds

Hi. What should be the type annotation of `inputs` for steps that join `foreach` outcomes? I have mypy complaining about it:
`error: Function is missing a type annotation for one or more arguments  [no-untyped-def]`
function:
```@step
def join_something(self, inputs) -> None:
    self.merge_artifacts(inputs)

    self.next(self.run_something_else)```

good point. It's <https://github.com/Netflix/metaflow/blob/master/metaflow/datastore/inputs.py#L1|this >`Inputs`<https://github.com/Netflix/metaflow/blob/master/metaflow/datastore/inputs.py#L1| object>, which we should include in the public docs

it supports iteration, looking up by index, and looking up by step name, which is useful with static names branches, so you can do `inputs.branch_a.x` and `inputs.branch_b.x`