[ROCm][Bugfix] Fix GQA=8 crash in mla_decode_fwd_impl by padding nhead to AITER minimum (gfx942/gfx950)

[psakhamo]

Purpose

Fix crash in _rocm_aiter_mla_decode_fwd_impl when nhead=8 (GLM-5.1 at TP=8 on ROCm).
Resolves ROCm/aiter#2821 at the vLLM layer as a stopgap until native ASM kernels land.

Root cause

aiter.mla_decode_fwd dispatches to mla_decode_stage1_asm_fwd. For nhead=8 in
non-persistent mode, no matching ASM binary exists — the kernel faults at C-level
with a NULL pointer dereference (Memory access fault on address nil).

Fix

When nhead < 16 and 16 % nhead == 0, pad q from 8→16 heads via repeat_interleave
before calling mla_decode_fwd, then extract real heads from o afterwards.
Mirrors SGLang nsa_backend.py::_forward_aiter (same bug, same fix, see ROCm/aiter#2821).

Test Plan

• Layer 1 (basic inference): PASS
• Layer 2 (streaming tool-call finish_reason=tool_calls): PASS
• SWE-Bench agent run (harbor/swe-agent, sympy task): 46-47 turns completed,
  agent submits patches. No GPU faults during run.
• Context extended: ~13k → ~20k tokens

Test Result

Before: crash with Memory access fault / SRAM ECC after ~13k tokens.
After: 46-47 agent turns, no GPU faults.

Note: ~20k ceiling remains in ASM binary — native gqa=8 kernel (ROCm/aiter#2821)
needed for full fix.

Related: ROCm/aiter#2821, vllm-project#36855