[BugFix][Relax] Add structural_equal verification to subroutine cache lookup

[3em0]

Summary

• SubroutineMixin._get_subroutine() used structural_hash as the sole cache key without structural_equal verification. If two different arg_sinfo values produced the same 64-bit hash (collision), the cache would return a previously compiled function with mismatched parameter shapes, leading to silently incorrect compiled output.
• Changed the cache to store a list of (arg_sinfo, result) pairs per hash bucket and verify with structural_equal on lookup, consistent with the pattern in block_builder.cc.
• Added a security advisory document and regression test.

Root Cause

The subroutine cache (cls._gvar) was keyed by (structural_hash(arg_sinfo), is_dataflow). A hash match was treated as proof of structural equality, skipping the necessary structural_equal check. This is a hash-only lookup anti-pattern — hash determines the bucket, but equality must confirm the match.

For comparison, block_builder.cc correctly uses StructuralHash + StructuralEqual together as the hash and equality functions for std::unordered_map.

Test plan

• Existing test test_linear passes (no regression)
• New test test_different_shapes_produce_distinct_subroutines passes — verifies that the same Module class with different input shapes generates distinct subroutines

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request addresses a correctness issue in the subroutine cache of SubroutineMixin._get_subroutine() where relying solely on structural_hash could lead to incorrect function reuse. The fix implements a bucketed cache that verifies hits using structural_equal. A security advisory and a regression test are also included. Feedback suggests further refining the cache lookup_key by including the function object to prevent collisions between different implementations with identical input signatures.