Support higher-order influence computation

[eb8680]

Our implementation of chirho.robust.ops.influence_fn estimates the first-order efficient influence function, but there are some settings where computing higher-order influence functions may enable additional bias correction (e.g. https://arxiv.org/abs/0805.3040 or https://arxiv.org/abs/2305.04116 ).

As with ordinary derivatives, the discussion in "Higher Order Tangent Spaces and Influence Functions" suggests it should be possible to compute higher-order influence functions and correction terms by applying a first-order influence estimator to itself recursively:

This recursion should not be difficult to implement using our existing machinery, although nesting our naive Monte Carlo gradient estimators may not be computationally feasible beyond order 4 or so.

Tasks:

• Change type of influence_fn to better support self-composition Make influence_fn a higher-order Functional #492
• Add unit tests and any necessary fixes for higher-order influence computation