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dagster

Hi Team,  I’m trying to use MonthlyPartitionsDefinition with multi_asset and it doesn’t go well with mypy, here is an sample
```from dagster import AssetOut, MonthlyPartitionsDefinition, multi_asset

@multi_asset(
    partitions_def=MonthlyPartitionsDefinition(start_date='2022-01-01'),
    outs={'test': AssetOut()},
)
def test_multi_asset():
    pass```

Mypy error output is
```test_asset.py:5: error: Argument "partitions_def" to "multi_asset" has incompatible type "MonthlyPartitionsDefinition"; expected "Optional[PartitionsDefinition[object]]"  [arg-type]```

I think we should update the type for  `partitions_def` to `Optional[PartitionsDefinition[Any]]`

Hi Sean, thanks for bringing this to our attention-- I think we will solve this by making the TypeVar for `PartitionsDefinition` covariant. Look for this in the next release.