Add serialization function for TF LSTM cell.

There are few issues we will need to address in future.

1. TF RNN cell currently accept callable only as activation function, while keras allows string and convert to activation function. This cause a weird backward dependency from tf.nn.op to tf.keras. It will be nice to have a unified place for activation functions within TF.
2. There is a name conflict for few cells between Keras and TF, namely LSTMCell and GRUCell. The serial/de-serialization need special care since the tf.nn.rnn_cell is not visible to Keras.

Update TF LSTM Cell to support proper serialization.

PiperOrigin-RevId: 208217949