fix: handle negative exponents in integer powi - #2043

Follow-up to tracel-ai/burn#6355, where @laggui asked to fix signed exponent handling in powi_int and keep the wrapping calculation in a powi_unsigned helper, so Burn can pick the right one per dtype.

Changes

• powi_int read its i32 exponent as u32, so 3^-1 gave -1431655765 (the inverse of 3 mod 2^32). A negative exponent now gives 1 / base^|exp| truncated toward zero: 1 for a base of 1, 1 or -1 by parity for a base of -1, and 0 otherwise, including a base of 0. That matches Burn's Flex backend and torch.pow on integer tensors. Non-negative exponents are unchanged.
• For an unsigned base, only 1 stays 1 under a negative exponent. Its maximum value isn't treated as -1.
• The wrapping calculation is now the public powi_unsigned(base, exp: Vector<u32, N>). Callers with an unsigned exponent keep powers above i32::MAX.

This changes integer powi on every backend that lowers through powi_int (WGSL, SPIR-V, C++, LLVM): a negative exponent used to act like a large unsigned one.

Testing

• New test_powi_int_negative_exponent fails on main (Values differ: actual=-1431655765, expected=0, index=0) and passes with the fix. test_powi_int now expects truncated results for its negative-exponent cases; on main it fails at (3i64, i32::MIN).
• New test_powi_unsigned covers exponents above i32::MAX for 8- to 64-bit bases.
• On Windows with an RTX 5060 Ti, cargo test -p cubecl-wgpu --lib (WGSL, 699 passed) and the same with --features spirv (949 passed) both pass. I couldn't run CUDA, HIP, Metal or the CPU runtime.
• cargo fmt --check and cargo clippy -p cubecl-core --all-targets --features export_tests -- -D warnings are clean. I didn't run the full cargo xtask validate.
• With this commit on top of the cubecl rev Burn pins, Burn's int powi tests pass on the Vulkan backend after switching PowiOp to powi_unsigned for unsigned dtypes and powi_int for signed ones. Without that switch, a u32 exponent above i32::MAX gives 0, so the Burn change has to land together with the rev bump.

Validate your PR with burn.

• Create a new branch or fork of the cubek repo
• Update the main Cargo.toml with this PR hash.
• Fix any broken tests or compilation errors in cubek.
• Submit a PR in cubek with your fixes and link it here.
• Create a new branch or fork of the burn repo
• Update the main Cargo.toml with this PR and the cubek PR hash.
• Fix any broken tests or compilation errors in burn.
• Submit a PR in burn with your fixes and link it here.

I used Claude Code to write this change.