input: Write a python function to find the length of the linked list.
  assert length_of_linked_list(llist) == 4
  
  llist2 = LinkedList()
  llist2.push(10)
  llist2.push(20)
  assert length_of_linked_list(llist2) == 2
  
  llist3 = LinkedList()
  assert length_of_linked_list(llist3) == 0
output: class Node:
      def __init__(self, data):
          self.data = data
          self.next = None
  
  class LinkedList:
      def __init__(self):
          self.head = None
  
      def push(self, new_data):
          new_node = Node(new_data)
          new_node.next = self.head
          self.head = new_node
  
  def length_of_linked_list(llist):
      count = 0
      current = llist.head
      while current:
          count += 1
          current = current.next
      return count
  
  # Test cases
  llist = LinkedList()
  llist.push(1)
  llist.push(2)
  llist.push(3)
  llist.push(4)
  assert length_of_linked_list(llist) == 4
  
  llist2 = LinkedList()
  llist2.push(10)
  llist2.push(20)
  assert length_of_linked_list(llist2) == 2
  
  llist3 = LinkedList()
  assert length_of_linked_list(llist3) == 0