Hi @zahraabbasian ,
I just tried running your code, and I think that the loss indeed decreases. (It fluctuates a lot [e.g. at .2x] and looks increasing for some time, but if you wait long enough, we can see it is slowly decreasing [e.g. to .18x]) . In the interactive log, I saw the progress bar stop quite early, so maybe you can wait for 1 epoch to conclude.
Moreover, since you use TF, it’s pretty straightforward to use TPU which can give at least 4x speed boost in Colab. This Kaggle notebook 18 shows a very concise way to efficiently train/predict Huggingface’s  XLMRoberta  (which is the same format as  Roberta ) . Hope it help!