Thank you very much!!! I will read the  source code  tommorrow because is too late.Thanks again for your patient !！
But I still can’t understand why the gradient descent was zero.
I see the document
last_hidden_state (tf.Tensor of shape (batch_size, sequence_length, hidden_size)) – Sequence of hidden-states at the output of the last layer of the model.
It is the parameters of bert after all,why Dense(FC) can’t match  them and do the gradient descent?