Implement PartitionSHAP attribution method

[sisird864]

Motivation:

Captum currently provides KernelShap and ShapleyValueSampling, but lacks a
fast hierarchical Shapley estimator.
PartitionSHAP (Lundberg & Erion 2021) reduces the number of model
evaluations from O(M · d) to O(M · log d) by
recursively partitioning the feature set. Users working with large‐parameter
LLMs and computer-vision models will benefit from 10-100× faster attributions
without sacrificing accuracy.

---

Proposed API (mirrors Captum style):

from captum.attr._core.perturbation_attribution import PerturbationAttribution
from typing import Callable, Any, Optional

class PartitionShap(PerturbationAttribution):
    def __init__(
        self,
        forward_func: Callable,
        baselines: Optional[Any] = None,
        max_evals: int = 512,
        perturbations_per_eval: int = 1,
        partition_agg: str = "mean",   # mean | sum | max
        cluster_method: str = "greedy" # future-proof for alt heuristics
    ): ...

---

Returns → tensor of Shapley values with same shape as inputs.

Design outline:

1. Hierarchy builder – greedy divisive split on absolute gradients

   • (torch.topk for speed, stop at single-feature leaves).

2. Conditional expectation – Monte-Carlo permutations per internal node,

   • batched using perturbations_per_eval.

3. Reuse Captum helpers

   • _tensorize_baselines, _construct_perturbations
   • infer_target_device_dtype for GPU / dtype safety.

4. Early-exit guard – stop once eval_count ≥ max_evals, emit UserWarning.

Acceptance criteria / test plan

• Correctness:
  PartitionSHAP ≈ KernelSHAP on a 5-feature linear model
  (torch.testing.assert_close(..., atol=1e-2, rtol=1e-2)).

• Speed:
  On bert-base-uncased CLS token, n_samples=512 runs ≥20× faster than
  KernelSHAP n_samples=2048 (CPU benchmark).

• Coverage ≥99 % for new file (pytest --cov=...).

• Docs:
  docs/source/algorithms.rst section
  Tutorial notebook: GPT-2 toxicity example (<3 min CPU runtime).

Task checklist:

• Core implementation (captum/attr/_core/partition_shap.py)
• Unit tests (tests/attr/test_partition_shap.py)
• Benchmarks script (optional, skipped on CI)
• API & math docs
• Example notebook
• Changelog entry

References:

G. Lundberg & G. Erion, “Partition SHAP: Explaining complex models via
recursive feature partitioning,” 2021.
(arXiv 2105.14814)

Assignees:

@sisird864 — claiming this feature implementation. Maintainers: please add the appropriate labels (triaged, feature, help-wanted) and let me know of any design concerns before I begin.
Thanks! Looking forward to contributing.
— @sisird864