Fix for loop copy warning

[jmackay2]

This fixes a warning seen building on Ubuntu 24.04.

Warning:

gtsam/gtsam/discrete/Assignment.h:93:21: error: loop variable ‘<structured bindings>’ creates a copy from type ‘const std::pair<long unsigned int, long unsigned int>’ [-Werror=range-loop-construct]
   93 |     for (const auto [idx, _] : keys) assignment[idx] = 0;  // Initialize from 0

This change uses a reference to avoid the copy.