Document cuDNN fallback for deterministic training (#7559)

# Description

Document `TORCH_CUDNN_V8_API_DISABLED=1` as an optional setting that may
improve run-to-run determinism for convolutional training workloads. The
note explains that the setting selects PyTorch's cuDNN v7 API instead of
cuDNN v8 execution plans.

No new dependencies are required.

## Type of change

- Documentation update

## Release backport

- [x] <!-- backport-active-release --> Backport this pull request to the
active release branch after it merges into `develop`

## Screenshots

Not applicable.

## Checklist

Docker and GPU tests run on demand. Push the commits you want tested,
then
comment `run-ci` on the pull request.

- [x] I have read and understood the [contribution
guidelines](https://isaac-sim.github.io/IsaacLab/main/source/refs/contributing.html)
- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`uv run isaaclab -f`
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works (not applicable to this documentation-only change)
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`source/<pkg>/changelog.d/` for every touched package (no source package
is touched)
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exists there

## Validation

- `uv run --isolated --extra dev --extra ov -- make -C docs
current-docs`
- `uv run isaaclab -f`

Author: kellyguo11

docs/source/features/reproducibility.rst

@@ -37,6 +37,12 @@ optional PyTorch deterministic algorithms. Whether the **rendering** half of the
 on the workload: **physics-only** simulation does not render at all; **RTX** rendering (non-minimal
 mode) needs it for reproducible imagery; **Newton** rendering is already deterministic.
 
+.. note::
+
+   For convolutional workloads, setting ``TORCH_CUDNN_V8_API_DISABLED=1`` before launching training
+   may improve run-to-run determinism by making PyTorch use the cuDNN v7 API instead of cuDNN v8
+   execution plans.
+
 **Physics determinism** comes from the same flag in the Isaac Lab RL training entrypoints, which
 set :attr:`~isaaclab.physics.PhysicsCfg.deterministic` on the backend resolved by presets. That
 field is the backend-agnostic request: each physics manager translates it into its own settings