PyTorch's superimposed gradients

gistfile1.txt

import torch
import torch.nn as nn
from torch.autograd import Variable
import torch.optim as optim

a = torch.ones(1,2)
b = torch.nn.Linear(2,1)
b.zero_grad()

c = b(Variable(a))
c.backward()
print(list(b.parameters())[0].grad)   # Pair of	1's

c = b(Variable(a))
c.backward()
print(list(b.parameters())[0].grad)   #	Pair of	2's; gradients were added

b.zero_grad()	
print(list(b.parameters())[0].grad)   #	Pair of	0's; gradients were reset
c = b(Variable(a))
c.backward()
print(list(b.parameters())[0].grad)   # Pair of	1's again