Hi,

I’m trying to fit a neural network using a training loop in Pytorch but I’m unable to calculate the validation error in the training loop because of an output-input size mismatch. The primary problem is that the size of the output in the validation set is always equal to the size of the output in the last stage of the training neural network and is not equal to the size of the input given by the validation data loader.

In the following code, the expected output size of prds is of length 40 but the code below gives prds of length 8 which is the size of y_pred in the last training loop. As a result the loss function doesn’t work because it gets one input of length 8 (prds) and one input of length 40 (y_val). I would be very gratful if someone could help me find a way to get prds of the correct length.

Note: If I run the validation set outside the entire training loop (i.e. after all the epochs are over) , the validation error calculated. Here is my code

```
net = MixedInputModel(emb_szs,len(contin_vars), 0.04, 1, [100,50], [0.0001,0.0001] ,y_range=y_range, use_bn=True, is_reg=True, is_multi=False)
loss = nn.MSELoss()
learning_rate = 1e-2
opt = optim.SGD(net.parameters(),lr = learning_rate,momentum = 0.9, weight_decay = 1e-3)
scheduler = torch.optim.lr_scheduler.StepLR(opt,1)
```

```
for epoch in range(1):
losses,losses_val=[],[]
net.train()
dl = iter(md.trn_dl)
for t in range(len(list(md.trn_dl))): #number of batches
l = next(dl)
x_cat, x_cont,y = l
#net.train()
#opt.zero_grad()
#a. Forward pass: compu
y_pred = net(V(x_cat),V(x_cont))
#print(y_pred)
ls = loss(y_pred, V(y))
losses.append(ls)
# b.Use the optimizer object to zero all of the gradients for the variables to be updated (which are the learnable weights of the model)
opt.zero_grad()
#c. Backward pass: compute gradient of the loss with respect to model parameters
ls.backward()
#d. Calling the step function on an Optimizer makes an update to its parameters
opt.step()
scheduler.step()
#validation loop
net.eval()
vali_dl = iter(md.val_dl)
for tt in range(len(list(md.val_dl))):
vdl = next(vali_dl)
xv_cat,xv_cont,y_val = vdl
prds = net(V(xv_cat),V(xv_cont))
ls_val = loss(prds, V(y_val))
losses_val.append(ls_val)
print(losses_val)
print(losses)
```