Update transformer_conv.py (#3538)

Initialise all attributes in layer for torch.jit.script compatability.

torch_geometric/nn/conv/transformer_conv.py

@@ -110,6 +110,7 @@ def __init__(
         self.concat = concat
         self.dropout = dropout
         self.edge_dim = edge_dim
+        self._alpha = None
 
         if isinstance(in_channels, int):
             in_channels = (in_channels, in_channels)