You can concatenate the tensors along the specific dimension.
Your question can be briefly expressed like below,
a = torch.Size(1, 3, 7)
b = torch.Size(1, 3, 7)

result = torch.cat((a, b), dim=1)

Then, you can get  the result tensor size of (1, 6, 7)
The sample code
for i in range(it):
    try:
        a = torch.cat((a, new_a), dim=1)
    except:
        a = new_a