keras - How to use MC Dropout on a variational dropout LSTM layer on keras?

Question

I'm currently trying to set up a (LSTM) recurrent neural network with Keras (tensorflow backend). I would like to use variational dropout with MC Dropout on it. I believe that variational dropout is already implemented with the option "recurrent_dropout" of the LSTM layer but I don't find any way to set a "training" flag to put on to true like a classical Dropout layer.