Design doc for per-block gradient checkpointing (KI-3): the no-tape forward +
recompute-on-backward design, the `checkpoint` primitive, per-block wrapping,
the exactness/correctness argument (same kernels + inputs → identical grads),
composition with bf16+DDP+batched, and the verification plan (on-vs-off grad
gate + memory/throughput before→after, dim1024-fits). Bench table left as TBD
to fill after the dash5 run.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Wrap each transformer block's forward in the checkpoint primitive when
recompute is enabled (Phase T13 / KI-3). To make the block forward a pure
segment fn (no `&self` borrow, so it can re-run in the backward closure),
extract the block body + its helpers (linear / norm_gamma / attention /
swiglu_mlp) into free functions parameterised by (cfg, compute_dtype) and add
`Block::block_params()` (the 11 leaves in the params() per-block order). The
non-recompute path calls `block_forward` directly — identical graph to before.
- `TinyTransformer::with_recompute(bool)` builder (opt-in; default off keeps the
unchanged tape / bit-identical numerics).
- `--recompute` flag wired into bin/train and bin/train_ddp (DDP: each rank
checkpoints independently).
Correctness gate: tests/recompute.rs builds two identical models (recompute
on/off), runs the same batched loss+backward, and asserts the forward logits,
the loss, and EVERY parameter grad match within tight fp tol — parameterised
over fp32 and bf16 (T12 composition).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
