input: Write a function to replace a given string with another string in the input sentence. If the string to be replaced does not exist in the sentence, return the original sentence.
  
  assert replace_word("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach"
  assert replace_word("I love swimming on the beach", "swimming", "running")=="I love running on the beach"
  assert replace_word("I love swimming on the beach", "dancing", "running")=="I love swimming on the beach"
  assert replace_word("I love swimming on the beach", "swimming", "")=="I love  on the beach"
  assert replace_word("", "swimming", "running")==""
output: def replace_word(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)