Optional forward mode for jac/jac-vec products

@amontoison observed that we could use forward mode for the jacobian and jacobian-vector products when the number of variables is less than the number of rows to get better performance than the current approach that uses reverse mode always.

The issue does not name files or tests. Start by locating the jacobian and jacobian-vector product implementations and their reverse-mode differentiation path, then inspect existing tests or benchmarks for these products. Done means forward mode is selected when variables outnumber? the rows? Verify the issue's stated condition carefully, preserve correctness, and demonstrate the intended performance improvement.

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