input: Write a python function to extract the subject and the main verb from a sentence using SpaCy.
  assert extract_subject_verb("The cat sat on the mat.") == "cat sat"
  assert extract_subject_verb("She is reading a book.") == "She is"
  assert extract_subject_verb("John loves Mary.") == "John loves"
output: import spacy
  
  def extract_subject_verb(sentence):
      nlp = spacy.load("en_core_web_sm")
      doc = nlp(sentence)
      subject = ""
      verb = ""
      
      for token in doc:
          if token.dep_ == "nsubj":
              subject = token.text
          if token.dep_ == "ROOT":
              verb = token.text
      
      return f"{subject} {verb}"