[AutoDiff] Nuke `AutoDiffParameterIndices`. (#27514)
Replace the last usages of `AutoDiffParameterIndices` (in `DifferentiableAttr`
and `DifferentiatingAttr`) with `AutoDiffIndexSubset`.
Delete `AutoDiffParameterIndices`.
Delete `AutoDiffParameterIndicesBuilder`; directly use `AutoDiffIndexSubset::get`
instead, constructing temporary `llvm::SmallBitVector` when necessary.
Rewrite parameter indices utilities to use `AutoDiffIndexSubset`:
- `autodiff::getLoweredParameterIndices`
- `autodiff::getSubsetParameterTypes `
Resolves TF-538.
TF-874 tracks improving the implementation of `AutoDiffIndexSubset` helpers:
see issue for details.
