neural network - What is num_units in tensorflow BasicLSTMCell?

Question

In MNIST LSTM examples, I don't understand what "hidden layer" means. Is it the imaginary-layer formed when you represent an unrolled RNN over time?

Why is the num_units = 128 in most cases ?

Answer 1

From this brilliant article

num_units can be interpreted as the analogy of hidden layer from the feed forward neural network.The number of nodes in hidden layer of a feed forward neural network is equivalent to num_units number of LSTM units in a LSTM cell at every time step of the network.

See the image there too!