Training a tensorflow model with an intermediate function call in training loop

I am trying to train a simple neural network where the input data is taken from a matlab simulink simulation and the output is then fed back into a different matlab simulink simulation. My code is as follows:

import tensorflow as tf
from tensorflow import keras
from tensorflow.keras import layers
import matplotlib.pyplot as plt
import numpy as np
import pandas as pd
import random


def get_pid_values():
    # call simulink model that just produces PID values

    return random.random()


def get_plant(intermediate_val):
    # get plant output.
    return random.random()


class CustomDataGen(tf.keras.utils.Sequence):
    
    def __init__(self, df, X_col,
                 batch_size,
                 input_size=(1,),
                 shuffle=True):
        
        self.df = df.copy()
        self.X_col = X_col
        self.batch_size = batch_size
        self.input_size = input_size
        self.shuffle = shuffle
        
        self.n = len(self.df)
    
    def __get_input(self, index):
        # Need to adjust this to support retrieving ref voltage.
        return self.df[self.X_col].iloc[index]
    
    def on_epoch_end(self):
        if self.shuffle:
            self.df = self.df.sample(frac=1).reset_index(drop=True)
    
    def __getitem__(self, index):
        X = self.__get_input(index)
        return X
    
    def __len__(self):
        return self.n // self.batch_size


def get_model(input_shape, hidden, output_shape):
    inputs = keras.layers.Input(shape=input_shape)
    x = layers.Dense(hidden, activation="relu")(inputs)
    x = layers.Dense(hidden, activation='relu')(x)
    outputs = layers.Dense(output_shape)(x)
    model = keras.Model(inputs=inputs, outputs=outputs, name="pid-modifier")
    return model


loss_object = tf.keras.losses.MeanSquaredError()

def loss(y_ref, y_plant):
  y_ = y_plant
  y = y_ref
  return loss_object(y_true=y, y_pred=y_)


if __name__ == "__main__":

    # Hyperparameters
    lr = 0.01
    num_epochs = 1
    hidden_size = 4
    net_input_size = 1
    net_output_size = 1
    batch_size = 1
    reference_fpath = "Run2_rThrottleTarget.csv"

    references = pd.read_csv(reference_fpath)

    data_generator = CustomDataGen(df=references, X_col='Throttle', batch_size=1)

    # Keep results for plotting
    train_loss_results = []


    # Initialize optimizer
    optimizer = tf.keras.optimizers.Adam(learning_rate=lr)

    # error intitial condition
    err = 0

    # instantiate model
    model = get_model(input_shape=(2,), hidden=hidden_size, output_shape=net_output_size)

    for epoch in range(num_epochs):

        for ref in data_generator:

            with tf.GradientTape() as tape:
                tape.watch(model.trainable_variables)
                
                # Get pid values
                pid = get_pid_values()
                
                # Group ref with pid voltage for input
                input = tf.constant([[ref, pid]])
                
                # Get the adjusted voltage from the network
                intermediate_val = model(input)

                # Get the plant output based on the adjusted value.
                plant = get_plant(intermediate_val)

                plant = tf.constant([plant], dtype=tf.float64)
                ref = tf.constant([ref], dtype=tf.float64)
                
                # Calculate loss 
                loss_value = loss(ref, plant)

            grads = tape.gradient(loss_value, model.trainable_weights)
                
            optimizer.apply_gradients(zip(grads, model.trainable_weights))

            err = ref - plant


        if epoch % 50 == 0:
            print("Epoch {:03d}: Loss: {:.3f}".format(epoch, loss_value))
    

    fig, axes = plt.subplots(1, figsize=(12, 8))
    fig.suptitle('Training Metrics')

    axes[0].set_ylabel("Loss", fontsize=14)
    axes[0].plot(train_loss_results)

    plt.show()

For the moment I am just mocking the calls to simulink by returning a random number. My problem is that when I take the model output and then call the function that mocks a call to simulink and calculate my loss:

# Get the adjusted voltage from the network
intermediate_val = model(input)

# Get the plant output based on the adjusted value.
plant = get_plant(intermediate_val)

plant = tf.constant([plant], dtype=tf.float64)
ref = tf.constant([ref], dtype=tf.float64)

# Calculate loss 
loss_value = loss(ref, plant)

I get the error ValueError: No gradients provided for any variable. I’ve figured out that if I pass the model’s output directly to the loss function everything works fine. My question is how can I have the intermediate step of passing my model’s output to another function and using the returned value to calculate loss?