Hello, everyone

I try to run LSTM for multivariate time series data

But the source code below causes error

which says

RuntimeError: Trying to backward through the graph a second time, but the buffers have already been freed. Specify retain_graph=True when calling backward the first time.

Source code is as below

Thank you in advance

```
class LSTMModel(torch.nn.Module):
def __init__(self,n_features,seq_length):
super(LSTMModel, self).__init__()
self.n_features = n_features
self.seq_len = seq_length
self.n_hidden = n_features # number of hidden states
self.n_layers = 1 # number of LSTM layers (stacked)
self.l_gru = torch.nn.LSTM(input_size = n_features,
hidden_size = self.n_hidden,
num_layers = self.n_layers,
batch_first = True)
self.l_linear = torch.nn.Linear(self.n_hidden*self.seq_len, 1)
self.hidden = self.init_hidden(batch_size)
def init_hidden(self, batch_size):
# even with batch_first = True this remains same as docs
hidden_state = torch.zeros(self.n_layers,batch_size,self.n_hidden)
cell_state = torch.zeros(self.n_layers,batch_size,self.n_hidden)
return (hidden_state, cell_state)
def forward(self, x):
batch_size, seq_len, _ = x.size()
lstm_out, self.hidden = self.l_gru(x,self.hidden)
x = lstm_out.contiguous().view(batch_size,-1)
return self.l_linear(x)
mv_net.train()
for t in range(train_episodes):
for b in range(0,len(X),batch_size):
inpt = X[b:b+batch_size,:,:]
target = y[b:b+batch_size]
x_batch = torch.tensor(inpt,dtype=torch.float32)
y_batch = torch.tensor(target,dtype=torch.float32)
#mv_net.init_hidden(x_batch.size(0))
output = mv_net(x_batch)
loss = criterion(output.view(-1), y_batch)
loss.backward()
optimizer.step()
optimizer.zero_grad()
print('step : ' , t , 'loss : ' , loss.item())
```