input: Write a function to replace all occurrences of a given character in a string with the provided replacement character. If the input string is empty, return the same empty string.
  
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXo WorXd!"
  assert replace_all_occurrences("Hello World!", 'o', 'Z')=="HellZ WZrld!"
  assert replace_all_occurrences("Hello World!", 'W', 'M')=="Hello Morld!"
  assert replace_all_occurrences("", 'a', 'b')==""
  assert replace_all_occurrences("Hello World!", '!', '?')=="Hello World?"
output: def replace_all_occurrences(input_str, target_char, replacement_char):
      return input_str.replace(target_char, replacement_char)