update noise scheduler api for ldm (#1417)

Fixes # .

### Description
update [noise scheduler
api](https://github.com/Project-MONAI/GenerativeModels/blob/b1c1a459c98a4d55570fe775fdcfad1707dae71b/generative/networks/schedulers/ddpm.py#L75)
for ldm.
corresponding
[PR](https://github.com/Project-MONAI/model-zoo/pull/449/files) in
bundle

### 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: Can-Zhao <[email protected]>
Co-authored-by: Yiheng Wang <[email protected]>

Author: Can-Zhao

generative/2d_ldm/train_diffusion.py

@@ -148,7 +148,7 @@ def main():
 
     scheduler = DDPMScheduler(
         num_train_timesteps=args.NoiseScheduler["num_train_timesteps"],
-        beta_schedule="scaled_linear",
+        schedule="scaled_linear_beta",
         beta_start=args.NoiseScheduler["beta_start"],
         beta_end=args.NoiseScheduler["beta_end"],
     )

generative/3d_ldm/train_diffusion.py

@@ -144,7 +144,7 @@ def main():
 
     scheduler = DDPMScheduler(
         num_train_timesteps=args.NoiseScheduler["num_train_timesteps"],
-        beta_schedule="scaled_linear",
+        schedule="scaled_linear_beta",
         beta_start=args.NoiseScheduler["beta_start"],
         beta_end=args.NoiseScheduler["beta_end"],
     )