Give ability to bind SparseVector

[AlexandreAmice]

I noticed that currently it is not possible to bind Drake methods which output a SparseVector because "there is no notion of compressed/uncompressed mode for a SparseVector".

This is a small fix attempting to fix this, but I am not familiar with this repository so don't know how to test this nor upgrade Drake with the fix.

cc @EricCousineau-TRI since you seem to be the maintainer

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This change is 

[EricCousineau-TRI]

Do you have example drake PR that shows this failing?

Also, if we'd want this to land, we'd want to add testing, ideally representative of what you encountered.

Reviewable status: 0 of 1 files reviewed, all discussions resolved

[jwnimmer-tri]

Drake policy re: this repository has changed over time. We have a goal of using vanilla upstream pybind11 (and eventually, nanobind). As such, we should strongly disprefer adding any patches here -- instead, we should add code into Drake that meets the need.

[jwnimmer-tri]

As such, we should strongly disprefer adding any patches here -- instead, we should add code into Drake that meets the need.

Well, I suppose really there are two valid ways forward:

(1) Add utility function(s) to Drake's pydrake headers to solve the problem.

(2) Contribute a change upstream https://github.com/pybind/pybind11 to solve the problem.

In any case, new pull requests into our fork won't be accepted, so I'll close this out.