[prototype] Cleaning up the size dimension methods (#6828)

* Cleaning up the size dimension methods.

* Change error messages.

torchvision/prototype/transforms/functional/_meta.py

@@ -7,7 +7,18 @@ from torchvision.prototype.features import BoundingBoxFormat, ColorSpace
 from torchvision.transforms import functional_pil as _FP, functional_tensor as _FT
 
 
-get_dimensions_image_tensor = _FT.get_dimensions
+def get_dimensions_image_tensor(image: torch.Tensor) -> List[int]:
+    chw = list(image.shape[-3:])
+    ndims = len(chw)
+    if ndims == 3:
+        return chw
+    elif ndims == 2:
+        chw.insert(0, 1)
+        return chw
+    else:
+        raise TypeError(f"Input tensor should have at least two dimensions, but got {ndims}")
+
+
 get_dimensions_image_pil = _FP.get_dimensions
 
 
@@ -24,7 +35,17 @@ def get_dimensions(image: Union[features.ImageTypeJIT, features.VideoTypeJIT]) -
         return get_dimensions_image_pil(image)
 
 
-get_num_channels_image_tensor = _FT.get_image_num_channels
+def get_num_channels_image_tensor(image: torch.Tensor) -> int:
+    chw = image.shape[-3:]
+    ndims = len(chw)
+    if ndims == 3:
+        return chw[0]
+    elif ndims == 2:
+        return 1
+    else:
+        raise TypeError(f"Input tensor should have at least two dimensions, but got {ndims}")
+
+
 get_num_channels_image_pil = _FP.get_image_num_channels
 
 
@@ -36,11 +57,11 @@ def get_num_channels(image: Union[features.ImageTypeJIT, features.VideoTypeJIT])
     if isinstance(image, torch.Tensor) and (
         torch.jit.is_scripting() or not isinstance(image, (features.Image, features.Video))
     ):
-        return _FT.get_image_num_channels(image)
+        return get_num_channels_image_tensor(image)
     elif isinstance(image, (features.Image, features.Video)):
         return image.num_channels
     else:
-        return _FP.get_image_num_channels(image)
+        return get_num_channels_image_pil(image)
 
 
 # We changed the names to ensure it can be used not only for images but also videos. Thus, we just alias it without
@@ -49,8 +70,12 @@ get_image_num_channels = get_num_channels
 
 
 def get_spatial_size_image_tensor(image: torch.Tensor) -> List[int]:
-    width, height = _FT.get_image_size(image)
-    return [height, width]
+    hw = list(image.shape[-2:])
+    ndims = len(hw)
+    if ndims == 2:
+        return hw
+    else:
+        raise TypeError(f"Input tensor should have at least two dimensions, but got {ndims}")
 
 
 @torch.jit.unused