Add examples of Multi-weight support + model usage (#6013)

* Adding code examples for image classification + quant

* Adding code example detection

* Adding code example segmentation

* Adding code example for video classification

* Adding information on how to use the new API.

* Putting back the comma.

* Apply suggestions from code review
Co-authored-by: Nicolas Hug <contact@nicolas-hug.com>

* Remove output to avoid staleness from flakiness.

* Minor fixes.
Co-authored-by: Nicolas Hug <contact@nicolas-hug.com>

docs/source/models_new.rst

@@ -24,6 +24,49 @@ keypoint detection, video classification, and optical flow.
     `documentation 
     <https://pytorch.org/docs/stable/notes/serialization.html#id6>`_   
 
+As of v0.13, TorchVision offers a new `Multi-weight support API
+<https://pytorch.org/blog/introducing-torchvision-new-multi-weight-support-api/>`_ for loading different weights to the
+existing model builder methods:
+
+.. code:: python
+
+    from torchvision.models import resnet50, ResNet50_Weights
+
+    # Old weights with accuracy 76.130%
+    resnet50(weights=ResNet50_Weights.IMAGENET1K_V1)
+
+    # New weights with accuracy 80.858%
+    resnet50(weights=ResNet50_Weights.IMAGENET1K_V2)
+
+    # Best available weights (currently alias for IMAGENET1K_V2)
+    # Note that these weights may change across versions
+    resnet50(weights=ResNet50_Weights.DEFAULT)
+
+    # Strings are also supported
+    resnet50(weights="IMAGENET1K_V2")
+
+    # No weights - random initialization
+    resnet50(weights=None)  # or resnet50()
+
+
+Migrating to the new API is very straightforward. The following method calls between the 2 APIs are all equivalent:
+
+.. code:: python
+
+    from torchvision.models import resnet50, ResNet50_Weights
+
+    # Using pretrained weights:
+    resnet50(weights=ResNet50_Weights.IMAGENET1K_V1)
+    resnet50(pretrained=True)  # deprecated
+    resnet50(True)  # deprecated
+
+    # Using no weights:
+    resnet50(weights=None)
+    resnet50(pretrained=False)  # deprecated
+    resnet50(False)  # deprecated
+
+Note that the ``pretrained`` parameter is now deprecated, using it will emit warnings and will be removed on v0.15.
+
 
 Classification
 ==============
@@ -56,6 +99,34 @@ weights:
    models/vision_transformer
    models/wide_resnet
 
+|
+
+Here is an example of how to use the pre-trained image classification models:
+
+.. code:: python
+
+    from torchvision.io import read_image
+    from torchvision.models import resnet50, ResNet50_Weights
+
+    img = read_image("test/assets/encode_jpeg/grace_hopper_517x606.jpg")
+
+    # Step 1: Initialize model with the best available weights
+    weights = ResNet50_Weights.DEFAULT
+    model = resnet50(weights=weights)
+    model.eval()
+
+    # Step 2: Initialize the inference transforms
+    preprocess = weights.transforms()
+
+    # Step 3: Apply inference preprocessing transforms
+    batch = preprocess(img).unsqueeze(0)
+
+    # Step 4: Use the model and print the predicted category
+    prediction = model(batch).squeeze(0).softmax(0)
+    class_id = prediction.argmax().item()
+    score = prediction[class_id].item()
+    category_name = weights.meta["categories"][class_id]
+    print(f"{category_name}: {100 * score:.1f}%")
 
 Table of all available classification weights
 ---------------------------------------------
@@ -78,6 +149,35 @@ pre-trained weights:
    models/googlenet_quant
    models/mobilenetv2_quant
 
+|
+
+Here is an example of how to use the pre-trained quantized image classification models:
+
+.. code:: python
+
+    from torchvision.io import read_image
+    from torchvision.models.quantization import resnet50, ResNet50_QuantizedWeights
+
+    img = read_image("test/assets/encode_jpeg/grace_hopper_517x606.jpg")
+
+    # Step 1: Initialize model with the best available weights
+    weights = ResNet50_QuantizedWeights.DEFAULT
+    model = resnet50(weights=weights, quantize=True)
+    model.eval()
+
+    # Step 2: Initialize the inference transforms
+    preprocess = weights.transforms()
+
+    # Step 3: Apply inference preprocessing transforms
+    batch = preprocess(img).unsqueeze(0)
+
+    # Step 4: Use the model and print the predicted category
+    prediction = model(batch).squeeze(0).softmax(0)
+    class_id = prediction.argmax().item()
+    score = prediction[class_id].item()
+    category_name = weights.meta["categories"][class_id]
+    print(f"{category_name}: {100 * score}%")
+
 
 Table of all available quantized classification weights
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -101,6 +201,37 @@ pre-trained weights:
    models/fcn
    models/lraspp
 
+|
+
+Here is an example of how to use the pre-trained semantic segmentation models:
+
+.. code:: python
+
+    from torchvision.io.image import read_image
+    from torchvision.models.segmentation import fcn_resnet50, FCN_ResNet50_Weights
+    from torchvision.transforms.functional import to_pil_image
+
+    img = read_image("gallery/assets/dog1.jpg")
+
+    # Step 1: Initialize model with the best available weights
+    weights = FCN_ResNet50_Weights.DEFAULT
+    model = fcn_resnet50(weights=weights)
+    model.eval()
+
+    # Step 2: Initialize the inference transforms
+    preprocess = weights.transforms()
+
+    # Step 3: Apply inference preprocessing transforms
+    batch = preprocess(img).unsqueeze(0)
+
+    # Step 4: Use the model and visualize the prediction
+    prediction = model(batch)["out"]
+    normalized_masks = prediction.softmax(dim=1)
+    class_to_idx = {cls: idx for (idx, cls) in enumerate(weights.meta["categories"])}
+    mask = normalized_masks[0, class_to_idx["dog"]]
+    to_pil_image(mask).show()
+
+
 Table of all available semantic segmentation weights
 ----------------------------------------------------
 
@@ -130,6 +261,41 @@ weights:
    models/ssd
    models/ssdlite
 
+|
+
+Here is an example of how to use the pre-trained object detection models:
+
+.. code:: python
+
+
+    from torchvision.io.image import read_image
+    from torchvision.models.detection import fasterrcnn_resnet50_fpn_v2, FasterRCNN_ResNet50_FPN_V2_Weights
+    from torchvision.utils import draw_bounding_boxes
+    from torchvision.transforms.functional import to_pil_image
+
+    img = read_image("test/assets/encode_jpeg/grace_hopper_517x606.jpg")
+
+    # Step 1: Initialize model with the best available weights
+    weights = FasterRCNN_ResNet50_FPN_V2_Weights.DEFAULT
+    model = fasterrcnn_resnet50_fpn_v2(weights=weights, box_score_thresh=0.9)
+    model.eval()
+
+    # Step 2: Initialize the inference transforms
+    preprocess = weights.transforms()
+
+    # Step 3: Apply inference preprocessing transforms
+    batch = [preprocess(img)]
+
+    # Step 4: Use the model and visualize the prediction
+    prediction = model(batch)[0]
+    labels = [weights.meta["categories"][i] for i in prediction["labels"]]
+    box = draw_bounding_boxes(img, boxes=prediction["boxes"],
+                              labels=labels,
+                              colors="red",
+                              width=4, font_size=30)
+    im = to_pil_image(box.detach())
+    im.show()
+
 Table of all available Object detection weights
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -191,6 +357,38 @@ pre-trained weights:
 
    models/video_resnet
 
+|
+
+Here is an example of how to use the pre-trained video classification models:
+
+.. code:: python
+
+
+    from torchvision.io.video import read_video
+    from torchvision.models.video import r3d_18, R3D_18_Weights
+
+    vid, _, _ = read_video("test/assets/videos/v_SoccerJuggling_g23_c01.avi")
+    vid = vid[:32]  # optionally shorten duration
+
+    # Step 1: Initialize model with the best available weights
+    weights = R3D_18_Weights.DEFAULT
+    model = r3d_18(weights=weights)
+    model.eval()
+
+    # Step 2: Initialize the inference transforms
+    preprocess = weights.transforms()
+
+    # Step 3: Apply inference preprocessing transforms
+    batch = preprocess(vid).unsqueeze(0)
+
+    # Step 4: Use the model and print the predicted category
+    prediction = model(batch).squeeze(0).softmax(0)
+    label = prediction.argmax().item()
+    score = prediction[label].item()
+    category_name = weights.meta["categories"][label]
+    print(f"{category_name}: {100 * score}%")
+
+
 Table of all available video classification weights
 ---------------------------------------------------