Events recorded during CUDA graph capture cannot be queried with
cuEventElapsedTime after graph replay — the graph creates internal
copies. Reverted to original mega-graph capture structure.
Sub-graph timing fields kept in PhaseEvents for future use with
split-graph diagnostic mode or nsys profiling.
Also kept submit_loss_and_grad_ops() extraction and pub(crate) visibility
on launch_cublas_forward/backward for future per-phase graph splitting.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
3b2bf8d993