The AITER gluon preshuffle kernel (_gluon_deepgemm_fp8_paged_mqa_logits_
preshuffle) performs unmasked buffer_store writes up to ~190 float32
elements past context_length in each logits row when block_size=64.
With the previous exact-size allocation those stores corrupt the logits
of the adjacent row, causing wrong top-k selection and degenerate output.

Fix: introduce _get_paged_logits_buffer that allocates (rows,
cols + _PAGED_LOGITS_COL_PADDING) where _PAGED_LOGITS_COL_PADDING=256.
A non-contiguous [:rows, :cols] slice is intentionally avoided:
deepgemm_fp8_paged_mqa_logits assumes contiguous output and would compute
incorrect row offsets from a non-contiguous tensor. The full contiguous
allocation ensures stride(0) = cols + 256 consistently; the padding
columns absorb the OOB writes. top_k_per_row_decode takes logits.stride(0)
and logits.stride(1) as explicit arguments and bounds iteration by
seq_lens, so the extra columns are never read.

A fresh allocation per call (no global cache) ensures each HIP graph
bucket owns its own stable tensor pointer; a shared global reallocated
for a larger bucket would leave earlier-captured graphs with dangling
pointers on replay.

Also fixes device="cuda" -> q_fp8.device so TP ranks > 0 allocate on
the correct GPU.

Validated: GSM8K 5-shot flexible-extract 0.9416 on TP4 with HIP graphs
and block_size=64 (reference fork: 0.9409).

Related: vllm-project#40643 (maeehart: same padding with caching, draft pending MAF
investigation at num_speculative_tokens=2).

Co-authored-by: Markus Hartikainen <mahartik@amd.com>
Signed-off-by: Frida Andersson <fanderss@amd.com>