[docs] Move a sentence from `nn.Transformer` to `nn.TransformerEncoder` (#78337)
`nn.Transformer` is not possible to be used to implement BERT, while `nn.TransformerEncoder` does. So this PR moves the sentence 'Users can build the BERT model with corresponding parameters.' from `nn.Transformer` to `nn.TransformerEncoder`.
Fixes #68053
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78337
Approved by: https://github.com/jbschlosser
