LDM Script Fix (#1785)

Fixes # .

### Description
Fixes an old reference in one of the example LDM script files.

### Checks
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [ ] Avoid including large-size files in the PR.
- [ ] Clean up long text outputs from code cells in the notebook.
- [ ] For security purposes, please check the contents and remove any
sensitive info such as user names and private key.
- [ ] 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>`

Signed-off-by: Eric Kerfoot <eric.kerfoot@kcl.ac.uk>
Co-authored-by: YunLiu <55491388+KumoLiu@users.noreply.github.com>

Author: ericspod

generation/3d_ldm/inference.py

@@ -20,8 +20,8 @@
 import nibabel as nib
 import numpy as np
 import torch
-from generative.inferers import LatentDiffusionInferer
-from generative.networks.schedulers import DDPMScheduler
+from monai.inferers import LatentDiffusionInferer
+from monai.networks.schedulers import DDPMScheduler
 from monai.config import print_config
 from monai.utils import set_determinism