[Data] Make name a required argument for AggregateFn

[scottjlee]

Why are these changes needed?

Currently, name is an optional arg for AggregateFn, which can cause errors when the user defines a custom AggregateFn without a name. For example, here we will get an empty row due to the AggregateFn.name not being set.

This PR makes the name parameter required for AggregateFn, which resolves the issue.

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[bveeramani]

Did AggregateFn always error if name isn't specified? If there are cases where AggregateFn worked without a name, this'd be a breaking change.

[scottjlee]

all the existing constructor usages in ray specify a name. i think the error occurs when we call aggregate on a GroupedDataset without providing a name.

if there are external users using this without a name, it could potentially break, but i think throwing a TypeError specific for this input should suffice.

[raulchen]

Should also add a check. Changing the type hint doesn't have any effect at runtime.