[do-not-merge] providing cyclic layout support to enable Transformer evaluation

[copybara-service]

[do-not-merge] providing cyclic layout support to enable Transformer evaluation

Introduce end-to-end cyclic (bicyclic and tricyclic) layout support and
compiler optimizations required for evaluating Transformer models (e.g.
BERT-tiny SST-2) at 32k slots:

• In-layout strided reduce:

  • Support in-layout strided reductions along CRT-based cyclic layouts in
    ConvertToCiphertextSemantics without requiring physical layout conversion.
  • Lower reductions via binary span-doubling rotate-and-reduce operations
    followed by periodic replication of the valid prefix across tail slots.
  • Extend LayoutPropagation for linalg.reduce on cyclic layouts and upgrade
    partial target relations to canonical total congruences in elementwise
    rectification.

• Closed-form constant layout packing:

  • Add closed-form diagonal packing routines for rank-2 and rank-3
    cleartext weights (packBicyclicDiagonalClosedForm,
    packTricyclicDiagonalClosedForm, packDiagonalWeightClosedForm).
  • Bypass expensive polyhedral integer relation evaluation (Presburger /
    ISL) in AssignLayout, enabling instant constant packing for 32k/64k
    slot sizes.
  • Add fast paths in AssignLayout for splat constants and zero-filled pad
    buffers, and raise constant folding threshold to 65536.

• Shift network & rotation group optimizations:

  • Optimize conflict graph construction in ImplementShiftNetwork by grouping
    sources by slot position to eliminate redundant quadratic conflict edges.
  • Skip graph coloring entirely when no position collisions occur, safely
    defaulting unconflicted vertices to rotation group 0.

• Pipeline controls for large ciphertext sizes:

  • Introduce CommonPipelineOptions with --skip-layout-optimization,
    --layout-optimization-vve-tries, and --codegen-strategy flags wired
    through MlirToRLWE and PlaintextBackend pipelines.
  • Bypass expensive hour-scale Vos-Vos-Erkin (VVE) layout optimization cost
    evaluations at 32,768 slots where CRT-closed layouts are already optimal.

• Testing:

  • Add UtilsTest suites validating mathematical equivalence between
    closed-form diagonal packing formulas and Presburger relation evaluation.
  • Add lit tests for layout propagation and ciphertext semantics lowering of
    multidimensional and strided reductions.