[proto] Small improvement for tensor equalize op (#6738)

* [proto] Small improvement for tensor equalize op

* Fix code formatting

* Added a comment on the ops

torchvision/prototype/transforms/functional/_color.py

@@ -183,6 +183,30 @@ def autocontrast(inpt: features.InputTypeJIT) -> features.InputTypeJIT:
         return autocontrast_image_pil(inpt)
 
 
+def _scale_channel(img_chan: torch.Tensor) -> torch.Tensor:
+    # TODO: we should expect bincount to always be faster than histc, but this
+    # isn't always the case. Once
+    # https://github.com/pytorch/pytorch/issues/53194 is fixed, remove the if
+    # block and only use bincount.
+    if img_chan.is_cuda:
+        hist = torch.histc(img_chan.to(torch.float32), bins=256, min=0, max=255)
+    else:
+        hist = torch.bincount(img_chan.view(-1), minlength=256)
+
+    nonzero_hist = hist[hist != 0]
+    step = torch.div(nonzero_hist[:-1].sum(), 255, rounding_mode="floor")
+    if step == 0:
+        return img_chan
+
+    lut = torch.div(torch.cumsum(hist, 0) + torch.div(step, 2, rounding_mode="floor"), step, rounding_mode="floor")
+    # Doing inplace clamp and converting lut to uint8 improves perfs
+    lut.clamp_(0, 255)
+    lut = lut.to(torch.uint8)
+    lut = torch.nn.functional.pad(lut[:-1], [1, 0])
+
+    return lut[img_chan.to(torch.int64)]
+
+
 def equalize_image_tensor(image: torch.Tensor) -> torch.Tensor:
     if image.dtype != torch.uint8:
         raise TypeError(f"Only torch.uint8 image tensors are supported, but found {image.dtype}")
@@ -194,15 +218,9 @@ def equalize_image_tensor(image: torch.Tensor) -> torch.Tensor:
     if image.numel() == 0:
         return image
     elif image.ndim == 2:
-        return _FT._scale_channel(image)
+        return _scale_channel(image)
     else:
-        return torch.stack(
-            [
-                # TODO: when merging transforms v1 and v2, we can inline this function call
-                _FT._equalize_single_image(single_image)
-                for single_image in image.view(-1, num_channels, height, width)
-            ]
-        ).view(image.shape)
+        return torch.stack([_scale_channel(x) for x in image.view(-1, height, width)]).view(image.shape)
 
 
 equalize_image_pil = _FP.equalize