Changes `DPOptimizerClass` to generically accept and use any `dp_sum_query`.

[copybara-service[bot]]

Changes DPOptimizerClass to generically accept and use any dp_sum_query.

This enables creation of generic DPOptimizers by user's passing queries. The most common Gaussian query is automatically performed for convenience and backwards compatibility.

Byproducts of this update: -ensures consistent implementations between the internal (and legacy) get_gradients and newer _compute_gradients for all queries. -refactors for python readability.