Add tutorial to demonstrate how to train a generative model that can …

…generate large image volumes (Project-MONAI#1745)

Fixes Project-MONAI#1744.

### Description
Add tutorial to demonstrate how to train a generative model that can
generate large image volumes.

### 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: dongyang0122 <[email protected]>
Signed-off-by: Dong Yang <[email protected]>
Co-authored-by: Dong Yang <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

generative/maisi/configs/config_maisi_diff_model.json

@@ -0,0 +1,42 @@
+{
+    "noise_scheduler": {
+        "_target_": "generative.networks.schedulers.DDPMScheduler",
+        "num_train_timesteps": 1000,
+        "beta_start": 0.0015,
+        "beta_end": 0.0195,
+        "schedule": "scaled_linear_beta",
+        "clip_sample": false
+    },
+    "diffusion_unet_train": {
+        "batch_size": 1,
+        "cache_rate": 0,
+        "lr": 0.0001,
+        "n_epochs": 1000
+    },
+    "diffusion_unet_inference": {
+        "dim": [
+            128,
+            128,
+            128
+        ],
+        "spacing": [
+            1.0,
+            1.25,
+            0.75
+        ],
+        "top_region_index": [
+            0,
+            1,
+            0,
+            0
+        ],
+        "bottom_region_index": [
+            0,
+            0,
+            1,
+            0
+        ],
+        "random_seed": 0,
+        "num_inference_steps": 10
+    }
+}

generative/maisi/configs/environment_maisi_diff_model.json

@@ -0,0 +1,11 @@
+{
+    "data_base_dir": "./data",
+    "embedding_base_dir": "./embeddings",
+    "json_data_list": "./dataset.json",
+    "model_dir": "./models",
+    "model_filename": "diff_unet_ckpt.pt",
+    "output_dir": "./predictions",
+    "output_prefix": "unet_3d",
+    "trained_autoencoder_path": "./models/autoencoder_epoch273.pt",
+    "existing_ckpt_filepath": null
+}