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