Clean up independent feature masking in FeatureAblation

Summary: Holy jeez why didn't I just parameterize the tests. Anyway this last diff completely removes the behavior where feature IDs are treated independently across different tensors in `feature_mask`. More context on motivation for new cross-tensor attribution behavior in D74588409.

Reviewed By: cyrjano

Differential Revision: D83110168

fbshipit-source-id: be0e4e578e78b9ad49bc046e3b33184b79833ce9

Author: sarahtranfb