input: Write a python function to tokenize sentences using NLTK word_tokenize and count word frequencies.
  assert count_word_frequencies(["This is a test.", "This is another test."]) == {'This': 2, 'is': 2, 'a': 1, 'test': 2, '.': 2, 'another': 1}
  assert count_word_frequencies(["Hello world!", "Hello again, world!"]) == {'Hello': 2, 'world': 2, '!': 2, 'again': 1, ',': 1}
output: def count_word_frequencies(sentences):
      from nltk.tokenize import word_tokenize
      from collections import Counter
      
      all_words = []
      for sentence in sentences:
          words = word_tokenize(sentence)
          all_words.extend(words)
      
      return dict(Counter(all_words))