Lookup for `dict`s is causing uproot backend function to fail

[masonproffitt]

I'm seeing basically the same problem as #148 with ToFourMomentum, which I use in func-adl-uproot to create vector objects:

self = <func_adl.type_based_replacement.remap_by_types.<locals>.type_transformer object at 0x7f016eee1400>
node = <ast.Attribute object at 0x7f016f56fe50>

    def visit_Attribute(self, node: ast.Attribute) -> Any:
        t_node = self.generic_visit(node)
        assert isinstance(t_node, ast.Attribute)
        # If this is a dict reference, then figure out what the
        # type is for that value of the dict.
        if isinstance(t_node.value, ast.Dict):
            key = t_node.attr
            key_index = [
                e for e, k in enumerate(t_node.value.keys) if k.value == key  # type: ignore
            ]
            if len(key_index) == 0:
                if t_node.attr.lower() == "zip":
                    return t_node
>               raise ValueError(f"Key {key} not found in dict expression!!")
E               ValueError: Key ToFourMomentum not found in dict expression!!

This is coming from something like:

{
    'pt': Electron_pt,
    'eta': Electron_eta,
    'phi': Electron_phi,
    'mass': Electron_mass,
}.ToFourMomentum()

I think I use other functions in a pattern like this (not just Zip() and ToFourMomentum()), and I can imagine this would be a common type of thing for backends to add, so probably this should not raise an error and be a warning at most...

[masonproffitt]

Or maybe it should just skip trying to look things up if the Attribute is a function call? I think that's really what the problem is.