Replace prv-accountant with Opacus built-in PRV accountant

[andreatgretel]

Priority Level

Medium (Nice to have)

Is your feature request related to a problem?

The prv-accountant PyPI package (v0.2.0, Aug 2022) is effectively unmaintained - no release in 4 years. Its internal domain-size computation can overflow or hang for small noise multipliers (high epsilon), which #351 works around with a SIGALRM timeout and RDP fallback. Relying on an abandoned dependency for privacy accounting is a maintenance risk.

Describe the solution you'd like

Migrate from prv-accountant to Opacus's built-in PRV accountant (opacus.accountants.prv). Opacus ships a separate PRV implementation based on the same paper, and its compute_safe_domain_size uses Opacus's own RDP internally - which may avoid the hang since Opacus RDP handles high epsilon fine. This would also drop the prv-accountant dependency entirely.

Describe alternatives you've considered

• Keep the current approach from fix(dp): fall back to RDP accountant when PRV overflows or hangs #351 (SIGALRM timeout + RDP fallback). Works, but leaves the unmaintained dependency in place and means RDP's looser bounds are used silently when PRV fails.
• Vendor/fork prv-accountant and fix the overflow. Higher maintenance burden than switching to Opacus's actively maintained implementation.

Additional context

Discussion: #351 (comment)