input: Write a python function to find the synonym of a given word using the nltk library.
  assert get_synonym("happy") == ["felicitous", "glad", "happy", "well-chosen"]
  assert get_synonym("sad") == ["deplorable", "lamentable", "sad", "sorrowful"]
output: def get_synonym(word):
      from nltk.corpus import wordnet
      
      synonyms = set()
      
      for syn in wordnet.synsets(word):
          for lemma in syn.lemmas():
              synonyms.add(lemma.name())
      
      return list(synonyms)