Focal Loss Documentation enhanced (#5799)

* Update focal_loss.py

updated docstring for the `sigmoid_focal_loss`

* edited docstring of focal_loss

* Update focal_loss.py

* fix doc style.

* formatting

* formatting

* minor edits

* move attribution link
Co-authored-by: Vasilis Vryniotis <datumbox@users.noreply.github.com>

torchvision/ops/focal_loss.py

@@ -10,28 +10,28 @@ def sigmoid_focal_loss(
     alpha: float = 0.25,
     gamma: float = 2,
     reduction: str = "none",
-):
+) -> torch.Tensor:
     """
-    Original implementation from https://github.com/facebookresearch/fvcore/blob/master/fvcore/nn/focal_loss.py .
     Loss used in RetinaNet for dense detection: https://arxiv.org/abs/1708.02002.
 
     Args:
-        inputs: A float tensor of arbitrary shape.
+        inputs (Tensor): A float tensor of arbitrary shape.
                 The predictions for each example.
-        targets: A float tensor with the same shape as inputs. Stores the binary
+        targets (Tensor): A float tensor with the same shape as inputs. Stores the binary
                 classification label for each element in inputs
                 (0 for the negative class and 1 for the positive class).
-        alpha: (optional) Weighting factor in range (0,1) to balance
-                positive vs negative examples or -1 for ignore. Default = 0.25
-        gamma: Exponent of the modulating factor (1 - p_t) to
-               balance easy vs hard examples.
-        reduction: 'none' | 'mean' | 'sum'
-                 'none': No reduction will be applied to the output.
-                 'mean': The output will be averaged.
-                 'sum': The output will be summed.
+        alpha (float): Weighting factor in range (0,1) to balance
+                positive vs negative examples or -1 for ignore. Default: ``0.25``.
+        gamma (float): Exponent of the modulating factor (1 - p_t) to
+                balance easy vs hard examples. Default: ``2``.
+        reduction (string): ``'none'`` | ``'mean'`` | ``'sum'``
+                ``'none'``: No reduction will be applied to the output.
+                ``'mean'``: The output will be averaged.
+                ``'sum'``: The output will be summed. Default: ``'none'``.
     Returns:
         Loss tensor with the reduction option applied.
     """
+    # Original implementation from https://github.com/facebookresearch/fvcore/blob/master/fvcore/nn/focal_loss.py
     if not torch.jit.is_scripting() and not torch.jit.is_tracing():
         _log_api_usage_once(sigmoid_focal_loss)
     p = torch.sigmoid(inputs)