[Issue]: Potential Division by Zero in _paged_attention_kernel

Problem Description

https://github.com/ROCm/aiter/blob/d2f5f2791336a9839c65241b77a88f0a8407a054/csrc/cpp_itfs/pa/pa_kernels.cuh#L124-L125

The value of MTP_PARALLEL_THREADS is computed as:
https://github.com/ROCm/aiter/blob/d2f5f2791336a9839c65241b77a88f0a8407a054/csrc/cpp_itfs/pa/pa_kernels.cuh#L57-L60

If MTP_PARALLEL_THREADS > 4 (which can happen with large MTP values and certain GQA ratios),
then 4 / MTP_PARALLEL_THREADS = 0 due to integer division, causing division by zero and undefined behavior or a kernel crash.

Operating System

Ubuntu 22.04.5 LTS (Jammy Jellyfish)

CPU

AMD EPYC 9575F 64-Core Processor

GPU

8 x AMD Instinct MI355X

ROCm Version

ROCm version: 7.0.51831-a3e329ad8

ROCm Component

No response

Steps to Reproduce

Replicator script:
paged_attention_v1.issue2.mi355.py

Logs from running replicator script:
paged_attention_v1.issue2.mi355.log

(Optional for Linux users) Output of /opt/rocm/bin/rocminfo --support

No response

Additional Information

My goal is to enable CUDA graph support for speculative decoding in the ROCM_AITER_FA attention backend by adding spec_as_decode support and changing _cudagraph_support from UNIFORM_SINGLE_TOKEN_DECODE to UNIFORM_BATCH. This is expected to speed up speculative decoding: removes graph breaks and allows routing speculative tokens through the more efficient decode path instead of the extend path. The AITER PA API appears to support MTP>1, but the implementation seems to be buggy.

Start with csrc/cpp_itfs/pa/pa_kernels.cuh, especially the MTP_PARALLEL_THREADS calculation at lines 57-60 and the division at lines 124-125. Run paged_attention_v1.issue2.mi355.py with its accompanying log, then verify the paged-attention kernel handles large MTP values and relevant GQA ratios without division-by-zero behavior or a crash.