def sum_grads(grads_1, grads_2):
	"""
	Compute the sum of grads_1 and grads_2
	:param grads_1: dictionary of gradients, where the key is the index and the value the grad
	:param grads_2: dictionary of gradients, where the key is the index and the value the grad
	:return: a dictionary
	"""
	for idx, grad in grads_2.items():
		grads_1[idx] += grad
	return grads_1