Why rec encoder use EOS? not SOS

[Patickk]

hi:
I find there is no 'SOS' in code， I understand SOS should be embedding at the beginning.
Please tell me ,thanks!
---------------code-----------------------------------------------
class Encoder(nn.Module):
def init(self, hidden_dim, voc, char2id, id2char):
super(Encoder, self).init()
self.hidden_dim = hidden_dim
self.vocab_size = len(voc)
self.START_TOKEN = char2id['EOS']
self.emb = nn.Embedding(self.vocab_size, self.hidden_dim)
self.att = MultiHeadAttentionLayer(self.hidden_dim, 8)

def forward(self, x):
    batch_size, feature_dim, H, W = x.size()
    x_flatten = x.view(batch_size, feature_dim, H * W).permute(0, 2, 1)
    st = x.new_full((batch_size,), self.START_TOKEN, dtype=torch.long)
    emb_st = self.emb(st)
    holistic_feature, _ = self.att(emb_st, x_flatten, x_flatten)
    return