Enable torch-only NMOptimizer execution modes

[vedika-saravanan]

Description

This PR updates NMOptimizer to run through torch-backed contractions without relying on cuTensorNet. The optimizer now uses an internal reduced noise-contraction path so all supported execution modes share the same differentiable contraction flow.

Runtime / performance impact

This changes NMOptimizer execution to use torch-backed contractions and an internal reduced noise-contraction path. The reduced path avoids the full unreduced tensor-network contraction that can hit torch einsum label/memory limits at larger problem sizes.

Self-review checklist

Please confirm each item before requesting review. Check [x] or strike
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Before requesting review

• I reviewed my own full diff in GitHub or my editor.
• PR is in Draft if it is not yet ready for review.
• Temporary / debugging changes have been removed.
• Local test logs reviewed; no unexplained warnings or errors.
• CI logs reviewed; no unexplained warnings or errors.
• Full CI has been run.

Scope and size

• PR is under ~1000 lines, or an exception is justified in the description.
• Refactoring-only changes are isolated in their own PR(s).
• No existing tests were disabled or modified just to make this PR pass
  (if so, an issue has been raised).

Tests

• New functionality has new tests.
• Tests fail if the new functionality is broken (including crashes), not
  just when it is missing.
• Negative tests added where exceptions are expected.
• Truth data added where simple EXPECT_* / assert checks are
  insufficient for algorithmic correctness.
• CI runtime impact considered; team notified if significant.

Documentation

• Public-facing APIs have Doxygen docs.
• User-visible behavior changes have public docs, or a follow-up is
  tracked.

Code style

• Naming follows the existing convention (snake_case vs camelCase) for
  the area being modified.

Dependencies

• No new third-party dependencies, or the team has been notified and
  OSRB tickets filed.