##############################################################################
TODO: Implement this function using only a single loop over x_train.

You may not use torch.norm (or its instance method variant), nor any
functions from torch.nn or torch.nn.functional.
##############################################################################
Replace “pass” statement with your code
for i in range(num_train):
sum_=torch.sum((x_test-x_train[i,:]) **2,dim=1)

cists[i] = torch.sqrt(sum_)