Unmanaged gradient accumulation: ZeRO stage 3 support (#8217)
## Summary
- Extends unmanaged gradient accumulation
(`managed_gradient_accumulation=false`) to **ZeRO stage 3**
(non-offload). Stacked follow-up to #8203 (now merged).
- Stage 3 partitions gradients into `__param_id_to_grad_partition` on
every `backward()` (as in managed mode); `step()` only marks the
boundary via `finalize_gradient_accumulation_boundary()`.
- Since stage 3 is now permitted, also reject ZeRO **parameter** offload
(previously unreachable behind the stage-3 guard). ZeRO optimizer-state
offload, pipeline parallelism, DeepCompile, and Apex AMP remain
unsupported pending follow-ups.
## Test plan
Validated on a 2-GPU node:
- [x] Full `-k Unmanaged` suite (29 passed), including
`test_unmanaged_matches_managed[3]` and
`test_unmanaged_varying_backward_count[3]`
- [x] Stage-3 unmanaged feature subset (10 passed)
- [x] Core ZeRO-3 regression (`test_zero_grad_clip`,
`test_zero_leaf_module`, `test_zero_context`,
`test_zero_tensor_fragment`): 158 passed, 62 skipped (expected)
Docs (`config-json.md`, `training.rst`) updated; previewable on
`rtd-staging`.
Made with [Cursor](https://cursor.com)
Signed-off-by: Olatunji Ruwase <tunji.ruwase@snowflake.com>
Co-authored-by: Cursor <cursoragent@cursor.com>
