There is a typo in the doc and the PR 36 is going to fix it. You don’t need src_vocab and tgt_vocab to initiate the transformer module. Here is a simple example:
import torch
import torch.nn as nn
transformer_model = nn.Transformer(nhead=16, num_encoder_layers=12)
src = torch.rand((10, 32, 512))
tgt = torch.rand((20, 32, 512))
out = transformer_model(src, tgt)

A full example to apply nn.Transformer for the word language model could be found here 221. The example applies both RNN and transformer modules, respectively.