Cannot save Keras model when v2 behavior is disabled
Hi **@rsujeevan** ,
- I tried to run your code on colab using TF v2.15,2.16.1 and nightly faced the same issue. Please find the [gist](https://colab.sandbox.google.com/gist/Venkat6871/f1012cd57b392cf832f5441ea5012625/66985_2-15-2-16-1-v.ipynb) here for reference. 

Thank you!
### Issue type

Bug

### Have you reproduced the bug with TensorFlow Nightly?

No

### Source

binary

### TensorFlow version

2.15.1

### Custom code

Yes

### OS platform and distribution

Linux Ubuntu 20.04.5

### Mobile device

_No response_

### Python version

3.11.9

### Bazel version

_No response_

### GCC/compiler version

_No response_

### CUDA/cuDNN version

11.2/8.9

### GPU model and memory

_No response_

### Current behavior?

Since Tensorflow 2.13, I am unable to save Keras models when using graph execution in Tensorflow v1 behaviour. When I attempt to save, I get the following error:
```python-traceback
Traceback (most recent call last):
  File "/tf/ser_repro.py", line 13, in <module>
    model.compile(loss=loss, optimizer=used_optimizer)
  File "/tf/env/lib/python3.11/site-packages/tensorflow/python/trackable/base.py", line 204, in _method_wrapper
    result = method(self, *args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tf/env/lib/python3.11/site-packages/keras/src/engine/training_v1.py", line 321, in compile
    self._set_optimizer(optimizer)
  File "/tf/env/lib/python3.11/site-packages/keras/src/engine/training_v1.py", line 1473, in _set_optimizer
    self.optimizer = optimizers.get(optimizer)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tf/env/lib/python3.11/site-packages/keras/src/optimizers/__init__.py", line 306, in get
    return convert_to_legacy_optimizer(identifier)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tf/env/lib/python3.11/site-packages/keras/src/optimizers/__init__.py", line 239, in convert_to_legacy_optimizer
    config = optimizer.get_config()
             ^^^^^^^^^^^^^^^^^^^^^^
  File "/tf/env/lib/python3.11/site-packages/keras/src/optimizers/adam.py", line 211, in get_config
    "learning_rate": self._serialize_hyperparameter(
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tf/env/lib/python3.11/site-packages/keras/src/optimizers/optimizer.py", line 726, in _serialize_hyperparameter
    return hyperparameter.numpy()
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/tf/env/lib/python3.11/site-packages/tensorflow/python/ops/resource_variable_ops.py", line 690, in numpy
    raise NotImplementedError(
NotImplementedError: numpy() is only available when eager execution is enabled.
```

### Standalone code to reproduce the issue

```python
import tensorflow.compat.v1 as tf
tf.disable_v2_behavior()

import keras
from keras import layers

inp = keras.layers.Input(shape=(1,), name="input")
result = keras.layers.Dense(250, name="dense_layer")(inp)

model = keras.models.Model(inp, result, name="mdl")
loss = "categorical_crossentropy"
used_optimizer = keras.optimizers.Adam(learning_rate=0.01)
model.compile(loss=loss, optimizer=used_optimizer)

model.save("test-model.keras")
```


### Relevant log output

```shell
2024-05-05 05:31:41.834245: E external/local_xla/xla/stream_executor/cuda/cuda_dnn.cc:9261] Unable to register cuDNN factory: Attempting to register factory for plugin cuDNN when one has already been registered
2024-05-05 05:31:41.834286: E external/local_xla/xla/stream_executor/cuda/cuda_fft.cc:607] Unable to register cuFFT factory: Attempting to register factory for plugin cuFFT when one has already been registered
2024-05-05 05:31:41.836617: E external/local_xla/xla/stream_executor/cuda/cuda_blas.cc:1515] Unable to register cuBLAS factory: Attempting to register factory for plugin cuBLAS when one has already been registered
2024-05-05 05:31:41.846554: I tensorflow/core/platform/cpu_feature_guard.cc:182] This TensorFlow binary is optimized to use available CPU instructions in performance-critical operations.
To enable the following instructions: AVX2 AVX512F FMA, in other operations, rebuild TensorFlow with the appropriate compiler flags.
2024-05-05 05:31:42.773103: W tensorflow/compiler/tf2tensorrt/utils/py_utils.cc:38] TF-TRT Warning: Could not find TensorRT
WARNING:tensorflow:From /tf/env/lib/python3.11/site-packages/tensorflow/python/compat/v2_compat.py:108: disable_resource_variables (from tensorflow.python.ops.variable_scope) is deprecated and will be removed in a future version.
Instructions for updating:
non-resource variables are not supported in the long term
2024-05-05 05:31:43.608937: W tensorflow/core/common_runtime/gpu/gpu_device.cc:2256] Cannot dlopen some GPU libraries. Please make sure the missing libraries mentioned above are installed properly if you would like to use GPU. Follow the guide at https://www.tensorflow.org/install/gpu for how to download and setup the required libraries for your platform.
Skipping registering GPU devices...
Traceback (most recent call last):
  File "/tf/ser_repro.py", line 13, in <module>
    model.compile(loss=loss, optimizer=used_optimizer)
  File "/tf/env/lib/python3.11/site-packages/tensorflow/python/trackable/base.py", line 204, in _method_wrapper
    result = method(self, *args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tf/env/lib/python3.11/site-packages/keras/src/engine/training_v1.py", line 321, in compile
    self._set_optimizer(optimizer)
  File "/tf/env/lib/python3.11/site-packages/keras/src/engine/training_v1.py", line 1473, in _set_optimizer
    self.optimizer = optimizers.get(optimizer)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tf/env/lib/python3.11/site-packages/keras/src/optimizers/__init__.py", line 306, in get
    return convert_to_legacy_optimizer(identifier)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tf/env/lib/python3.11/site-packages/keras/src/optimizers/__init__.py", line 239, in convert_to_legacy_optimizer
    config = optimizer.get_config()
             ^^^^^^^^^^^^^^^^^^^^^^
  File "/tf/env/lib/python3.11/site-packages/keras/src/optimizers/adam.py", line 211, in get_config
    "learning_rate": self._serialize_hyperparameter(
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tf/env/lib/python3.11/site-packages/keras/src/optimizers/optimizer.py", line 726, in _serialize_hyperparameter
    return hyperparameter.numpy()
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/tf/env/lib/python3.11/site-packages/tensorflow/python/ops/resource_variable_ops.py", line 690, in numpy
    raise NotImplementedError(
NotImplementedError: numpy() is only available when eager execution is enabled.
```
