Clarify remaining uses of nondeterministic functions

[nh2]

@sandwichmaker In https://ceres-solver.googlesource.com/ceres-solver/+/36c73c26bb3fd9b319435aa9702a110678239d9c you write:

I have chosen not to delete the existing unstable implementations of these functions as they are used by the inner iteration minimizer.

Sometime in the near future I will clean up some of the duplicate code and see if we can move all the code to using a stable ordering.

In which part of the code are the unstable implementations still used? Grepping for ComputeSchurOrdering in commit 36c73c2, there seems to be no use of it besides in tests.

[nh2]

(Asking this because I've encountered nondeterminism when using the automatic inner iteration ordering.)

[nh2]

Hmm, I suspect it's this bit, am I right?

https://github.com/ceres-solver/ceres-solver/blob/01e23e3d33178fdd050973666505c1080cfe04c3/internal/ceres/parameter_block_ordering.cc#L111