feat(optim): add static Split-to-Slice rewrite

[xieofxie]

Summary

Add an opt-in static-split-to-slice algebraic rewrite that replaces eligible ONNX Split nodes with equivalent input-form Slice nodes while preserving output tensor names.

The rewrite only applies when the input shape, axis, and split boundaries are statically known. It supports explicit split sizes and equal splits, avoids malformed or dynamic cases, handles name collisions, preserves nested-subgraph captures, removes dead generated nodes and constants, and remains idempotent through the public optimization path.

Why this is not a pattern rewrite

The existing pattern framework is designed for fixed-topology replacements with compatible source and target schemas. Its match result exposes a single pattern output, and PatternRewriter constructs the replacement using that fixed output mapping.

A Split node is variadic: one matched node may have any number of outputs, and the replacement must generate one Slice node plus four boundary initializers for each output. The replacement topology and constants therefore depend on the specific matched node rather than on a fixed target pattern. Representing this correctly as a pattern would first require broader framework support for variadic output mappings and match-parameterized replacement topology.

For that reason, this transformation is implemented as a focused algebraic rewrite rather than adding model-specific behavior or forcing it into the current fixed-shape pattern abstraction.

[DingmaomaoBJTU]

Reviewed the static Split-to-Slice rewrite. Overall design looks solid: opt-in flag, conservative bail-out for dynamic/malformed cases, dead-code pruning, and nested-subgraph capture handling are all done carefully. Left two non-blocking inline notes.