Fix xPos numerical stability optimization

bp_transformers/layers/positional_encodings.py

@@ -230,7 +230,7 @@ class XPosEncoding(RotaryPositionalEncoding):
 
         dampening = (
             (torch.arange(0, d_model, 2) + gamma * d_model)
-            / (1.0 + gamma) * d_model
+            / ((1.0 + gamma) * d_model)
         ).repeat_interleave(2)
         self.register_buffer('dampening', dampening)