candle-core: add Device::enable_peer_access for cross-CUDA P2P transfers

[toddwbucy]

Fixes #3524.

Adds explicit `Device::enable_peer_access(&other)` so GPU-direct cross-card tensor operations (`Tensor::to_device(&other_cuda_device)`) succeed instead of erroring with `CUDA_ERROR_INVALID_CONTEXT` on first use.

What

• `CudaDevice::enable_peer_access(&self, other: &Self) -> Result<()>` — calls `cuCtxEnablePeerAccess` in both directions (`self ←→ other`), bound to the appropriate context for each direction.
• `Device::enable_peer_access(&self, other: &Self) -> Result<()>` — public surface, cuda-only (`#[cfg(feature = "cuda")]`); errors clearly if either side isn't a CUDA device.

Idempotent

• Same-ordinal pairs no-op, returning `Ok(())` — same-context "peer" access is meaningless.
• Repeat calls between the same context pair are safe — the driver's `CUDA_ERROR_PEER_ACCESS_ALREADY_ENABLED` (704) is folded into `Ok(())` inside the helper. So callers can put `enable_peer_access` on a hot path without worrying about state tracking.

Why explicit, not auto-enable in `BackendDevice::new`

Per #3524 I considered (A) explicit opt-in vs (B) opportunistic auto-enable in `BackendDevice::new`. Going with (A) here:

• Peer access is a real driver-state mutation with costs (UVA coordination on platforms that have it, no-op rejection on heterogeneous configs). Surfacing the opt-in keeps those visible.
• (B) requires global state — a registry of all already-constructed `CudaDevice`s — which is a much larger surgery and creates surprising side effects (every `Device::new_cuda(N)` would mutate driver state for every other extant CUDA device).
• (A) is forward-compatible with (B) — if you later want auto-enable, `BackendDevice::new` can just call this same helper.

Happy to switch to (B) instead if maintainers prefer that direction; the underlying `enable_peer_access_one_way` helper is the same either way.

Errors

Returns the underlying `DriverError` if the device pair doesn't support peer access (some heterogeneous or IOMMU-isolated configurations) or if either context is in a terminal state. Callers wanting to probe support before attempting can use `cuDeviceCanAccessPeer` separately; not wrapped here.

Validation

• `cargo build -p candle-core --features cuda` — clean
• Tested in a downstream project's topology bench (2× A6000 + NVLink, also tested over PCIe with NVLink bridge physically removed) where `Tensor::to_device(&other_cuda_device)` previously errored with `INVALID_CONTEXT`. With this patch + a one-time `encoder_device.enable_peer_access(&decoder_device)?` call before the first transfer, the cross-card path succeeds and routes through NVLink (or PCIe P2P).

Adjacent

This is the third small fix from our Jina V4 + multi-GPU work. The other two are #3520 (qwen2 RoPE fp32 cos/sin tables) and #3521 (FA v2.8.3 vendored kernel bump). All three were independently discovered while bringing a Jina V4 embedder up under candle on a 2-A6000 rig; happy to provide more context on any of them.