[Issue]: Potential Division by Zero in _paged_attention_kernel

[gronsti-amd]

Problem Description

aiter/csrc/cpp_itfs/pa/pa_kernels.cuh

Lines 124 to 125 in d2f5f27

	const int warp_mtp_idx = warpid / (4 / MTP_PARALLEL_THREADS);
	const int warp_row_idx = warpid % (4 / MTP_PARALLEL_THREADS);

The value of MTP_PARALLEL_THREADS is computed as:

aiter/csrc/cpp_itfs/pa/pa_kernels.cuh

Lines 57 to 60 in d2f5f27

	constexpr int MAX_ELEMENTS_PER_QUERY = DIVIDE_ROUND_UP(16, GQA_RATIO);
	constexpr int MTP_PER_THREAD = DIVIDE_ROUND_UP(MTP, MAX_ELEMENTS_PER_QUERY);
	constexpr int MTP_PARALLEL_THREADS = MTP / MTP_PER_THREAD;

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.