why language_model.py has different vectors

In language_model.py codes, class NextSentencePrediction and class MaskedLanguageModel has different input x in their forward function, In class NextSentencePrediction the input use "x[: 0] in "return self.softmax(self.linear(x[:, 0]))" but in class MaskedLanguageModel the input use x in "return self.softmax(self.linear(x))", I think if there is something wrong?

As I debug to this, both x in their has shape of (batch_size, seq_len, embedding_dim) like (64, 50, 256), we known this mains I have 64 sentences and each sentences has 50 words and each word is a 256 dim of vector. But the x[:, 0] means I take the first word in every 64 sentences so lead x[:, 0] to has shape of (64, 256). I don't understand why the task NextSentencePrediction should use this kind of input, can someone help me to explain this?