## Summary
- Route FPDT `SequenceChunk` and backward zero-chunk pins through
`get_accelerator().pin_memory()`, pinning only when the chunk is
on-accelerator (avoid pin-then-discard on CPU inputs).
- Route FastFileWriter AIO buffer through accelerator pin with
`make_copy=False`, and fall back to `Tensor.pin_memory()` when the CPU
accelerator torch path no-ops so DeepNVMe can still skip bounce buffers.
## Test plan
- [x] `pre-commit run --files` on touched paths (already run locally)
- [x] FPDT path smoke: `SequenceChunk` GPU pin + CPU reuse under native
and torch; `TestFPDTAttention` combo `[32-4-128-2048-4]` **PASSED**
- [x] Checkpoint FastFileWriter / AIO write with default torch backend:
`test_fast_file_writer_fd_close.py` **3 passed**; writer pin pattern
`is_pinned=True`
- [x] `DS_PIN_MEMORY_BACKEND=native` writer buffer: `is_pinned=True`,
`aio.is_pinned(buf)=True`, unpin OK; `test_pinned_manager.py` **5
passed**
Evidence: H200 autorun `job-20260819T184219Z` + `job-20260819T184528Z`
on `13131752` / `tjruwase/pin-memory-route-fpdt-writer` (follow-up EXIT
0). GitHub CI green.
Signed-off-by: Olatunji Ruwase <tunji.ruwase@snowflake.com>
Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: Ma, Guokai <guokai.ma@gmail.com>
Olatunji Ruwase
310eb8cb48