forget about masking when compute accuracy

[yinxusen]

cs230-code-examples/tensorflow/nlp/model/model_fn.py

Line 68 in 159df10

accuracy = tf.reduce_mean(tf.cast(tf.equal(labels, predictions), tf.float32))

The computation of accuracy and the metrics of accuracy below are lack of masking. In this way the accuracy could be wrong because of wrong predictions of padded tokens.