Fix post process function called in the instance segmentation example of mask2former (#34588)

* Fix post process function called in the instance segmentation example of mask2former

* fix description and additional notes for post_process_instance_segmentation of maskformers

* remove white space in maskformers post_process_instance_segmentation doc

* change image.size[::-1] to height and width for clarity in segmentation examples

docs/source/en/model_doc/rt_detr.md

@@ -57,7 +57,7 @@ Initially, an image is processed using a pre-trained convolutional neural networ
 >>> with torch.no_grad():
 ...     outputs = model(**inputs)
 
->>> results = image_processor.post_process_object_detection(outputs, target_sizes=torch.tensor([image.size[::-1]]), threshold=0.3)
+>>> results = image_processor.post_process_object_detection(outputs, target_sizes=torch.tensor([(image.height, image.width)]), threshold=0.3)
 
 >>> for result in results:
 ...     for score, label_id, box in zip(result["scores"], result["labels"], result["boxes"]):