Function flatten_join_inputs 

fn flatten_join_inputs(
    plan: LogicalPlan,
    possible_join_keys: &mut JoinKeySet,
    all_inputs: &mut Vec<LogicalPlan>,
    all_filters: &mut Vec<Expr>,
) -> Result<()>

Recursively accumulate possible_join_keys and inputs from inner joins (including cross joins).

Assumes can_flatten_join_inputs has returned true and thus the plan can be flattened. Adds all leaf inputs to all_inputs and join_keys to possible_join_keys