Add non-periodic grid gradient boundary mode

[loliverhennigh]

What changed

• adds boundary="one_sided" to rectilinear_grid_gradient and uniform_grid_gradient
• preserves boundary="periodic" as the default with the existing periodic code path unchanged
• computes arbitrary-grid one-sided finite-difference weights for first and second derivatives
• applies corrections across the full stencil radius, including the two edge points required by fourth-order uniform stencils
• supports torch and Warp forward paths and autograd; Warp retains its existing kernels for interior points and composes differentiable boundary corrections
• documents the new API and updates the changelog

Why

Periodic wrapping mixes values from opposite edges on bounded, non-periodic domains and corrupts boundary derivatives. Callers currently need to mask or replace those values themselves.

The explicit boundary mode is backward-compatible and leaves room for additional boundary treatments in the future.

Validation

• uv run --group dev python -m pytest -q test/nn/functional/derivatives
  • 202 passed
• uv run ruff check <changed Python files>
• uv run ruff format --check <changed Python files>
• verified public generated signatures include boundary: str = "periodic"

Tests cover analytic boundary derivatives, uniform/rectilinear equivalence, torch/Warp forward parity, torch/Warp backward parity, fourth-order stencil radius handling, invalid modes, and insufficient axis lengths.

Closes #1852.

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