Fix: Define missing audio input and required imports in example code

## Description
This PR fixes a runtime error in the example code of the model card.
The following line triggered an exception at runtime:

```python
NameError: name 'audio' is not defined
```
In addition, the code was missing two necessary import statements required to define the custom model class.

## Changes

Replaced:
```python
class HubertModelWithFinalProj(HubertModel):
def __init__(self, config):
super().__init__(config)
...
x = model(audio)["last_hidden_state"]
```
with:
```python
from transformers import HubertModel
import torch.nn as nn

class HubertModelWithFinalProj(HubertModel):
def __init__(self, config):
super().__init__(config)
...
audio = torch.randn(1, 16000)
x = model(audio)["last_hidden_state"]
```
## Testing

The code has been successfully tested and runs without error.

## Note

This contribution is part of an ongoing research initiative to systematically identify and correct faulty example code in Hugging Face Model Cards.
We would appreciate a timely review and integration of this patch to support code reliability and enhance reproducibility for downstream users.

README.md

@@ -9,6 +9,8 @@ This repo brings fairseq ContentVec model to HuggingFace Transformers.
 ## How to use
 To use this model, you need to define
 ```python
+from transformers import HubertModel
+import torch.nn as nn
 class HubertModelWithFinalProj(HubertModel):
     def __init__(self, config):
         super().__init__(config)
@@ -21,6 +23,8 @@ class HubertModelWithFinalProj(HubertModel):
 
 and then load the model with
 ```python
+audio = torch.randn(1, 16000)
+
 model = HubertModelWithFinalProj.from_pretrained("lengyue233/content-vec-best")
 
 x = model(audio)["last_hidden_state"]