Only applying the mask to the overlapping part of the tensors seems to work:
overlap = 540
lin_mask_part_1 = torch.linspace(0,1,overlap).repeat(1080,1).repeat(3,1,1).unsqueeze(0)
lin_mask_part_2 = torch.linspace(1,0,overlap).repeat(1080,1).repeat(3,1,1).unsqueeze(0)

test_tensor_1[:,:,:,overlap:] = test_tensor_1[:,:,:,overlap:] * lin_mask_part_2
test_tensor_2[:,:,:,:overlap] = test_tensor_2[:,:,:,:overlap] * lin_mask_part_1

base_tensor[:,:, 0:1080, 0:1080] = base_tensor[:,:, 0:1080, 0:1080] + test_tensor_1
base_tensor[:,:, 0:1080, overlap:overlap*3] = base_tensor[:,:, 0:1080, overlap:overlap*3] + test_tensor_2

ft = deprocess(base_tensor)
ft.save('part_faded_tensor.png')


part_faded_tensor1620×1080 995 KB