Fix focal loss tests (#8920)

Author: NicolasHug

torchvision/ops/focal_loss.py

@@ -33,7 +33,7 @@ def sigmoid_focal_loss(
     """
     # Original implementation from https://github.com/facebookresearch/fvcore/blob/master/fvcore/nn/focal_loss.py
 
-    if not (0 <= alpha <= 1) or alpha != -1:
+    if not (0 <= alpha <= 1) and alpha != -1:
         raise ValueError(f"Invalid alpha value: {alpha}. alpha must be in the range [0,1] or -1 for ignore.")
 
     if not torch.jit.is_scripting() and not torch.jit.is_tracing():