input: Write a function to generate n-grams of the given text with a specified window size. The function should return a Counter object containing the frequency of each n-gram. If the text is empty, the function should return an empty Counter.
  assert generate_ngrams("The quick brown fox jumps over the lazy dog", 3)==Counter({'the quick brown': 3, 'quick brown fox': 2, 'over the lazy': 1, 'brown fox jumps': 1, 'fox jumps over': 1, 'the lazy dog': 1, 'jumps over the': 1, 'quick brown fox jumps': 1})
  assert generate_ngrams("The quick brown fox", 2)==Counter({'the quick': 2, 'quick brown': 2, 'brown fox': 2})
  assert generate_ngrams("", 3)==Counter()
  assert generate_ngrams("a a a a a", 1)==Counter({'a': 5})
  assert generate_ngrams("a b", 3)==Counter()
output: def generate_ngrams(text, n):
      from collections import Counter
      if not text:
          return Counter()
      words = text.split()
      ngrams = [' '.join(words[i:i+n]) for i in range(len(words)-n+1)]
      return Counter(ngrams)