src/transformers/models/longformer/modeling_longformer.py/LongformerLMHead/forward
class LongformerLMHead:    def forward(self, features, **kwargs):
        x = self.dense(features)
        x = gelu(x)
        x = self.layer_norm(x)

        # project back to size of vocabulary with bias
        x = self.decoder(x)

        return x