[WIP] kda in torchtitan (vllm integration) - #4352
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(remaking bc i accideltny merged #4164 into my ghstack instead of main) this PR adds attention module for KDA (relying on attention gym KDA kernels and user facing APIs) **correctness** tested with c4 dataset: <img width="940" height="496" alt="Screenshot 2026-08-24 at 3 04 05 PM" src="https://github.com/user-attachments/assets/c07f5e37-688d-47a6-9e96-e8ea8493e72f" /> torchtitan attention gym vs hugging face (natural routing) ``` Full multimodal last-token logit parity (TorchTitan vs HuggingFace) KL=7.0921e-02 cos=0.927007 max_diff=1.6885e+00 top1=N top5=20% ``` ^i compared hugigng face and torchtitan layer by layer using identical weights and inputs, first divergence appears at block 2 where expert choice agreement was 94.5% and max router score diff was 0.0065. with nearly tied experts, small numerical differences led to different experts being in the top k which led to model executing different expert weights. if we force expert routing to be the same, we see more alignment. i did this by first running hugging face and recording the top k experts for every token at every layer and then replacing torchtitan with those choices. torchtitan attention gym vs hugging face (forced expert routing) ``` Full multimodal last-token logit parity (TorchTitan vs HuggingFace) KL=2.1806e-04 cos=0.999769 max_diff=9.6069e-02 top1=Y top5=80% ``` torchtitan fla vs hugging face ``` Full multimodal last-token logit parity (TorchTitan vs HuggingFace) KL=2.3385e-04 cos=0.999754 max_diff=9.7656e-02 top1=Y top5=80% ``` **performance** <img width="530" height="144" alt="Screenshot 2026-08-24 at 6 35 10 PM" src="https://github.com/user-attachments/assets/a880b257-1a21-4818-b148-e302125ff86c" /> ^attention gym is 1.38x and uses 26% less memory attention gym trace: <img width="1225" height="416" alt="Screenshot 2026-08-24 at 6 41 17 PM" src="https://github.com/user-attachments/assets/11950616-8467-4c85-9353-ebfaafdaf31b" /> fla trace: <img width="1246" height="323" alt="Screenshot 2026-08-24 at 6 41 49 PM" src="https://github.com/user-attachments/assets/6168e16c-a6f4-4e62-a5a9-848c7a02055d" /> Stack from [ghstack](https://github.com/ezyang/ghstack/tree/0.15.0) (oldest at bottom): * #4352 * __->__ #4351
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Stack from ghstack (oldest at bottom):