Provide the default vocab size for the transformer demo model (#18963)

src/lightning/pytorch/demos/transformer.py

@@ -26,7 +26,13 @@
 
 class Transformer(nn.Module):
     def __init__(
-        self, vocab_size: int, ninp: int = 200, nhead: int = 2, nhid: int = 200, nlayers: int = 2, dropout: float = 0.2
+        self,
+        vocab_size: int = 33278,  # default for WikiText2
+        ninp: int = 200,
+        nhead: int = 2,
+        nhid: int = 200,
+        nlayers: int = 2,
+        dropout: float = 0.2,
     ) -> None:
         super().__init__()
         self.pos_encoder = PositionalEncoding(ninp, dropout)