minor fix in controlnet flax example (#2986)

* fix the error when push_to_hub but not log validation

* contronet_from_pt & controlnet_revision

* add intermediate checkpointing to the guide

examples/controlnet/README.md

@@ -320,6 +320,12 @@ Then cd in the example folder and run
 pip install -U -r requirements_flax.txt
 ```
 
+If you want to use Weights and Biases logging, you should also install `wandb` now
+
+```bash
+pip install wandb
+```
+
 Now let's downloading two conditioning images that we will use to run validation during the training in order to track our progress
 
 ```
@@ -389,4 +395,17 @@ Note, however, that the performance of the TPUs might get bottlenecked as stream
 
 * [Webdataset](https://webdataset.github.io/webdataset/)
 * [TorchData](https://github.com/pytorch/data)
-* [TensorFlow Datasets](https://www.tensorflow.org/datasets/tfless_tfds)
+* [TensorFlow Datasets](https://www.tensorflow.org/datasets/tfless_tfds)
+
+When work with a larger dataset, you may need to run training process for a long time and it’s useful to save regular checkpoints during the process. You can use the following argument to enable intermediate checkpointing:
+
+```bash
+  --checkpointing_steps=500
+```
+This will save the trained model in subfolders of your output_dir. Subfolder names is the number of steps performed so far; for example: a checkpoint saved after 500 training steps would be saved in a subfolder named 500 
+
+You can then start your training from this saved checkpoint with 
+
+```bash
+   --controlnet_model_name_or_path="./control_out/500" 
+```

examples/controlnet/train_controlnet_flax.py

@@ -154,15 +154,16 @@ def log_validation(controlnet, controlnet_params, tokenizer, args, rng, weight_d
 
 def save_model_card(repo_id: str, image_logs=None, base_model=str, repo_folder=None):
     img_str = ""
-    for i, log in enumerate(image_logs):
-        images = log["images"]
-        validation_prompt = log["validation_prompt"]
-        validation_image = log["validation_image"]
-        validation_image.save(os.path.join(repo_folder, "image_control.png"))
-        img_str += f"prompt: {validation_prompt}\n"
-        images = [validation_image] + images
-        image_grid(images, 1, len(images)).save(os.path.join(repo_folder, f"images_{i}.png"))
-        img_str += f"![images_{i})](./images_{i}.png)\n"
+    if image_logs is not None:
+        for i, log in enumerate(image_logs):
+            images = log["images"]
+            validation_prompt = log["validation_prompt"]
+            validation_image = log["validation_image"]
+            validation_image.save(os.path.join(repo_folder, "image_control.png"))
+            img_str += f"prompt: {validation_prompt}\n"
+            images = [validation_image] + images
+            image_grid(images, 1, len(images)).save(os.path.join(repo_folder, f"images_{i}.png"))
+            img_str += f"![images_{i})](./images_{i}.png)\n"
 
     yaml = f"""
 ---
@@ -213,6 +214,17 @@ def parse_args():
         action="store_true",
         help="Load the pretrained model from a PyTorch checkpoint.",
     )
+    parser.add_argument(
+        "--controlnet_revision",
+        type=str,
+        default=None,
+        help="Revision of controlnet model identifier from huggingface.co/models.",
+    )
+    parser.add_argument(
+        "--controlnet_from_pt",
+        action="store_true",
+        help="Load the controlnet model from a PyTorch checkpoint.",
+    )
     parser.add_argument(
         "--tokenizer_name",
         type=str,
@@ -731,7 +743,10 @@ def main():
     if args.controlnet_model_name_or_path:
         logger.info("Loading existing controlnet weights")
         controlnet, controlnet_params = FlaxControlNetModel.from_pretrained(
-            args.controlnet_model_name_or_path, from_pt=True, dtype=jnp.float32
+            args.controlnet_model_name_or_path,
+            revision=args.controlnet_revision,
+            from_pt=args.controlnet_from_pt,
+            dtype=jnp.float32,
         )
     else:
         logger.info("Initializing controlnet weights from unet")
@@ -1021,6 +1036,8 @@ def main():
     if jax.process_index() == 0:
         if args.validation_prompt is not None:
             image_logs = log_validation(controlnet, state.params, tokenizer, args, validation_rng, weight_dtype)
+        else:
+            image_logs = None
 
         controlnet.save_pretrained(
             args.output_dir,