Fix NLST keypoint warping coordinates (#2073)

Fixes #2021 

### Description
This PR fixes the keypoint coordinate normalization used when sampling
the displacement field in `learn2reg_nlst_paired_lung_ct.ipynb`.

Previously, keypoint voxel coordinates were normalized using the image
dimension:

```python
offset = torch.as_tensor(fixed_image.shape[-3:]).to(fixed_keypoints.device) / 2
```

This corresponds to `2 * x / dim - 1`, which does not map voxel indices
`[0, dim - 1]` to `[-1, 1]`.

The normalization is updated to use `(dim - 1) / 2`, corresponding to:

```text
2 * x / (dim - 1) - 1
```

`align_corners=True` is also explicitly specified in `F.grid_sample` so
that the sampling convention is consistent with this normalization and
with MONAI `Warp`.

A small local `grid_sample` check confirmed that the updated
normalization maps voxel centers correctly.

### Checks
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applicable items -->
- [x] Avoid including large-size files in the PR.
- [x] Clean up long text outputs from code cells in the notebook.
- [x] For security purposes, please check the contents and remove any
sensitive info such as user names and private key.
- [x] Ensure (1) hyperlinks and markdown anchors are working (2) use
relative paths for tutorial repo files (3) put figure and graphs in the
`./figure` folder
- [ ] Notebook runs automatically `./runner.sh -t <path to .ipynb file>`

Local PEP 8/format checks passed with runner.sh. Notebook execution was
skipped by the existing runner skip pattern.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Corrected keypoint displacement sampling to align with PyTorch’s
corner-aligned coordinate system, improving registration accuracy and
consistency.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: mei <hmeixx@gmail.com>
Co-authored-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>

Author: hmeixx

3d_registration/learn2reg_nlst_paired_lung_ct.ipynb

@@ -660,11 +660,15 @@
     "    # warp vectors for keypoints (optional)\n",
     "    if fixed_keypoints is not None:\n",
     "        with torch.no_grad():\n",
-    "            offset = torch.as_tensor(fixed_image.shape[-3:]).to(fixed_keypoints.device) / 2\n",
+    "            offset = (torch.as_tensor(fixed_image.shape[-3:]).to(fixed_keypoints.device) - 1) / 2\n",
     "            offset = offset[None][None]\n",
     "            ddf_keypoints = torch.flip((fixed_keypoints - offset) / offset, (-1,))\n",
     "        ddf_keypoints = (\n",
-    "            F.grid_sample(ddf_image, ddf_keypoints.view(batch_size, -1, 1, 1, 3))\n",
+    "            F.grid_sample(\n",
+    "                ddf_image,\n",
+    "                ddf_keypoints.view(batch_size, -1, 1, 1, 3),\n",
+    "                align_corners=True,\n",
+    "            )\n",
     "            .view(batch_size, 3, -1)\n",
     "            .permute((0, 2, 1))\n",
     "        )\n",