input: Write a python function to generate the SHA-256 hash of a given string.
  assert generate_sha256('abc') == 'ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad'
  assert generate_sha256('hello world') == 'a948904f2f0f479b8f8197694b30184b0d2edaa4f14d3f68f2c8b1d5d9d5f7a7'
  assert generate_sha256('OpenAI') == 'd5fcab6e1f3e0cd59d7b4e2db4b84e0e2f5e0ed29aa64e0f7a2d5f92d79e59b8'
output: import hashlib
  
  def generate_sha256(input_string):
      sha256_hash = hashlib.sha256()
      sha256_hash.update(input_string.encode('utf-8'))
      return sha256_hash.hexdigest()