and suddenly solved , Although it looks no mistake but build_model function was caused,
I had manually defined that function in __init__   and this is solved
class Siamese(nn.Module):
    def __init__(self, vocab_size, embed_dim, hidden_dim):
        super().__init__()
        self.vocab_size = vocab_size
        self.embed_dim = embed_dim
        self.hidden_dim = hidden_dim
        self.fc = nn.Sequential(
            nn.Linear(self.hidden_dim, self.hidden_dim)
        )
        
        self.em = nn.Sequential(
        nn.Embedding(self.vocab_size, self.embed_dim),
        nn.LSTM(self.embed_dim, self.hidden_dim)
        )
        
    def forward(self, x1, x2):
        #n1
        lstm_out_1, _ = self.build_net(x1)
        print(lstm_out_1)
        lstm_out_1 = lstm_out_1.contiguous().view(-1, self.hidden_dim)
        l1 = self.fc(lstm_out_1)
        #n2
        lstm_out_2, _ = self.build_net(x2)
        lstm_out_2 = lstm_out_2.contiguous().view(-1, self.hidden_dim)
        l2 = self.fc(lstm_out_2)
        
        return f.normalize(l1), f.normalize(l2)
    
    def build_net(self, x):
        net = self.em(x)
        return net

but Why it was not working on previous code??