input: Write a python function to find the most frequent element in an array.
  assert most_frequent_element([3, 4, 6, 7, 3, 6, 8, 7, 6]) == 6
  assert most_frequent_element([1, 2, 2, 3, 3, 3, 4, 4, 4, 4]) == 4
  assert most_frequent_element([5, 5, 5, 1, 1, 2, 2, 2, 5, 5, 5]) == 5
output: def most_frequent_element(arr):
      from collections import Counter
      count = Counter(arr)
      return count.most_common(1)[0][0]