[TF 2.0] Signature for unranked tensor not working as intended
@MeghnaNatraj,
Could you please provide the simple standalone code to reproduce the issue and help us to analyse the issue in an effective way. Thank you! This issue is stale because it has been open for 7 days with no activity. It will be closed if no further activity occurs. Thank you.
<details><summary>Click to expand!</summary> 
 
 ### Issue Type

Bug

### Have you reproduced the bug with TF nightly?

Yes

### Source

source

### Tensorflow Version

2.14

### Custom Code

Yes

### OS Platform and Distribution

Linux

### Mobile device

_No response_

### Python version

3.9

### Bazel version

_No response_

### GCC/Compiler version

_No response_

### CUDA/cuDNN version

_No response_

### GPU model and memory

_No response_

### Current Behaviour?

trying to save a model where one of the functions in the model is defined as:

```
class XYZ(tf.keras.models.Model):

  @tf.function(input_signature=[tf.TensorSpec(shape=[None], dtype=tf.float32)])
  def coefficients(self, input_tensor: tf.Tensor):
  ....
  ....
```


When I train the model, and try to build it, I get the following error:

```
File "/build/work/83bb824f51c38f677e7ad84666ae2d5f4deb/google3/runfiles/google3/vr/perception/computational_photography/ml/video_enhancement/nets/hdrnet.py", [line 320](https://cs.corp.google.com/piper///depot/google3/vr/perception/computational_photography/ml/video_enhancement/nets/hdrnet.py?l=320&ws=mnatraj/3402&snapshot=40), in call  *
        grid, _ = self.coefficients(inputs[0])
    File "/build/work/83bb824f51c38f677e7ad84666ae2d5f4deb/google3/runfiles/google3/third_party/tensorflow/python/util/traceback_utils.py", [line 141](https://cs.corp.google.com/piper///depot/google3/third_party/tensorflow/python/util/traceback_utils.py?l=141&ws=mnatraj/3402&snapshot=40), in error_handler  **
        return fn(*args, **kwargs)
    File "/build/work/83bb824f51c38f677e7ad84666ae2d5f4deb/google3/runfiles/google3/third_party/tensorflow/python/eager/polymorphic_function/polymorphic_function.py", [line 820](https://cs.corp.google.com/piper///depot/google3/third_party/tensorflow/python/eager/polymorphic_function/polymorphic_function.py?l=820&ws=mnatraj/3402&snapshot=40), in __call__
        result = self._call(*args, **kwds)
    File "/build/work/83bb824f51c38f677e7ad84666ae2d5f4deb/google3/runfiles/google3/third_party/tensorflow/python/eager/polymorphic_function/polymorphic_function.py", [line 864](https://cs.corp.google.com/piper///depot/google3/third_party/tensorflow/python/eager/polymorphic_function/polymorphic_function.py?l=864&ws=mnatraj/3402&snapshot=40), in _call
        self._initialize(args, kwds, add_initializers_to=initializers)
    File "/build/work/83bb824f51c38f677e7ad84666ae2d5f4deb/google3/runfiles/google3/third_party/tensorflow/python/eager/polymorphic_function/polymorphic_function.py", [line 687](https://cs.corp.google.com/piper///depot/google3/third_party/tensorflow/python/eager/polymorphic_function/polymorphic_function.py?l=687&ws=mnatraj/3402&snapshot=40), in _initialize
        self._variable_creation_fn.get_concrete_function(args, kwds)
    File "/build/work/83bb824f51c38f677e7ad84666ae2d5f4deb/google3/runfiles/google3/third_party/tensorflow/python/eager/polymorphic_function/tracing_compiler.py", [line 182](https://cs.corp.google.com/piper///depot/google3/third_party/tensorflow/python/eager/polymorphic_function/tracing_compiler.py?l=182&ws=mnatraj/3402&snapshot=40), in get_concrete_function
        args, kwargs = function_type_utils.bind_function_inputs(
    File "/build/work/83bb824f51c38f677e7ad84666ae2d5f4deb/google3/runfiles/google3/third_party/tensorflow/python/eager/polymorphic_function/function_type_utils.py", [line 451](https://cs.corp.google.com/piper///depot/google3/third_party/tensorflow/python/eager/polymorphic_function/function_type_utils.py?l=451&ws=mnatraj/3402&snapshot=40), in bind_function_inputs
        raise TypeError(

    TypeError: Binding inputs to tf.function failed due to `Can not cast TensorSpec(shape=(1, 256, 256, 3), dtype=tf.float32, name=None) to TensorSpec(shape=(None,), dtype=tf.float32, name=None)`. Received args: (<tf.Tensor 'Placeholder:0' shape=(1, 256, 256, 3) dtype=float32>,) and kwargs: {} for signature: (input_tensor: TensorSpec(shape=(None,), dtype=tf.float32, name=None)).
```


Shouldn't specifying input signature as `None` imply that you can pass any input into this function? (unranked tensor)

### Standalone code to reproduce the issue

```shell
(no standalone code)
```


### Relevant log output

_No response_</details>