Reduce autotune with fusion panics for output position out of bounds

**Describe the bug** Fusion autotune panics for reduce: ``` 2026-05-29T17:21:58.972579Z INFO burn_train::learner::supervised::strategies::single::epoch: Executing training step for epoch 1 2026-05-29T17:21:58.972630Z INFO bunsen_preview_chat_dataloader::arrow::parquet: Loading Parquet: "/media/Data/nanochat/shard_00016.parquet" 2026-05-29T17:21:58.997351Z INFO burn_train::learner::supervised::strategies::single::epoch: Iteration 1 2026-05-29T17:21:59.001445Z INFO cubecl_runtime::tune::tune_cache: Load autotune cache ... 2026-05-29T17:21:59.001544Z INFO cubecl_runtime::tune::tune_cache: Loaded 0 autotune cached entries 2026-05-29T17:21:59.001556Z INFO cubecl_runtime::tune::tuner: Tuning FusedReduceAutotuneKey - ReduceKey: ReduceAutotuneKey { elem_input: Float(F32), elem_output: Float(F32), elem_acc: Float(F32), axis_is_contiguous: true, vector_size: 1024, vector_count: 4096 }, FuseNumReads: 1, FuseNumWrites: 8, FuseNumOps: 8 2026-05-29T17:21:59.136427Z INFO cubecl_runtime::tune::tune_cache: Load autotune cache ... 2026-05-29T17:21:59.136496Z INFO cubecl_runtime::tune::tune_cache: Loaded 0 autotune cached entries 2026-05-29T17:21:59.136503Z INFO cubecl_runtime::tune::tuner: Tuning ReduceAutotuneKey - ElemInput: Float(F32), ElemOutput: Float(F32), ElemAcc: Float(F32), AxisIsContiguous: true, VectorSize: 1024, VectorCount: 4096 2026-05-29T17:21:59.668041Z ERROR burn_train::learner::application_logger: PANIC => panicked at /home/crutcher/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/burn-cubecl-fusion-0.21.0/src/engine/codegen/ir.rs:500:63: index out of bounds: the len is 1 but the index is 1 ``` Somehow the registered output position is out of bounds in the global args? **To reproduce** * repo: <https://github.com/zspacelabs/bunsen> * cmd: `cargo run --release -p train-chat --features cuda -- --dataset-dir ~/Data/nanochat --batch-size 2 --shards 0..20` you have to pick a data dir you can drop some shards in. **Expected behavior** Should executed successfully. **Additional context** Originally reported [on discord](https://discord.com/channels/1038839012602941528/1091796857996451942/1509962795289149642).