coremltools does not export LSTM state?

## ❓Question

I am trying to convert an LSTM based model from tensorflow to CoreML to be used in a MacOS application. Despite of looking at hundreds of examples in documentation, and spending two days through all possible references to a similar problem, I can't find a solution. Here is a compact example that replicates the problem.

First we define a very simple LSTM model and export it to CoreML using coremltools:
```
import os
import tensorflow as tf
from tensorflow.keras import layers
from tensorflow import keras
import coremltools

model = keras.Sequential()
model.add(layers.Input(dtype="float32", batch_input_shape=(1,1,1)))
model.add(layers.LSTM(3,))
model.add(layers.Dense(1))
model.summary()

coreml_model_file = './test.mlpackage'

mlmodel = coremltools.convert(model,source="tensorflow")
mlmodel.save(coreml_model_file)
```
tensorflow version is 2.13.0, coremltools version is 6.3.0, python version 3.11. Tried with tensorflow 2.12.0, does not change a thing.

Dragging .mlpackage to Xcode performs perfect import. But I get the following as model specification: [SCREENSHOT OF MY MODEL](https://i.stack.imgur.com/HXQ0P.png)

Documentation (https://developer.apple.com/documentation/coreml/making_predictions_with_a_sequence_of_inputs?language=objc) suggests that the states of the LSTM layer should be exported as additional inputs and outputs, so the network can be applied to a new arbitrary sequence. Whatever I do I cannot get statein/stateout to appear in CoreML model.

What am I doing wrong? How do I export an LSTM model properly?

Tried different versions of tensorflow 2, coremltools. Stateful LSTM does not export at all. Changing input shape to (1, None, 1) does not help either.

Will really appreciate some help.

Start by running the compact TensorFlow-to-Core ML conversion example with the reported TensorFlow 2.13.0, coremltools 6.3.0, and Python 3.11 versions, then inspect the generated model specification in Xcode. Compare the result with Apple's sequence-input documentation and determine whether statein/stateout should be present. Done means identifying a supported export path or documenting the limitation and required conditions.

Written by the indexing model from the issue text.