feat: Support reduce top level function

[FBruzzesi]

What type of PR is this? (check all applicable)

• 💾 Refactor
• ✨ Feature
• 🐛 Bug Fix
• 🔧 Optimization
• 📝 Documentation
• ✅ Test
• 🐳 Other

Related issues

• Related issue [Enh]: Add support for nw.fold and nw.reduce #2554

Checklist

• Code follows style guide (ruff)
• Tests added
• Documented the changes

If you have comments or can explain your changes, please do so below

@MarcoGorelli , @dangotbanned I am opening a PR since I said I was working on this, but I am not particularly proud* of it to be honest.

In the lazy cases, there is a lot of code duplication, which also leads to reduce being poorly typed at the compliant level.

Now, if there was a way to unpack multi output expression (related to discord comment, the entire function would become:

def reduce(
    function: Callable[[Expr, Expr], Expr], exprs: IntoExpr | Iterable[IntoExpr]
) -> Expr:

    from functools import reduce as freduce

    if not exprs:
        msg = "At least one expression must be passed to `reduce`"
        raise ValueError(msg)
    if isinstance(exprs, Expr):
        exprs = [exprs]
    flat_exprs = flatten(exprs)

    return freduce(function, flat_exprs)

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*there is one exception to this statement which is the example which for once is not the usual horizontal sum 😅

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Edit: Clearly should have started with a draft