val_acc KeyError using ModelCheckpoint.

[raghav7203]

changing "{val_acc:.2f}" to "{val_accuracy:.2f}" and it will work fine,
as no variable named "val_acc" is defined but what you are trying to get is accuracy value after each epoch, which is probably defined as "val_accuracy".
@Omar178
I have commited the code change for the train_emotion_classifier.py. I have tested, it is working fine.
Please find below the pull request:

[raghav7203]

@Omar178 please verify for the pull request. It's much needed for the informative repositories like this.