Issue title: When running MultiHeadAttention I get "AttributeError: module 'keras' has no attribute 'applications"
 Issue body: When runnning the code I get the error: AttributeError: module 'keras' has no attribute 'applications'

**Version Info**

I am using Keras version 2.4.3

**Minimal Codes To Reproduce**

```
import keras
from keras.layers import Input
from keras_multi_head import MultiHeadAttention

print(keras.getversion())

input_layer =Input(
    shape=(2, 3),
    name='Input',
)
att_layer = MultiHeadAttention(
    head_num=3,
    name='Multi-Head',
)(input_layer)
model = keras.models.Model(inputs=input_layer, outputs=att_layer)
```

 Comments: 
 Comment 0: Is this still relevant? If so, what is blocking it? Is there anything you can do to help move it forward?

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.
 Comment 1: Fixed in `0.28.0`.
{'source': 'GithubIssues', 'repo_name_with_owner': 'CyberZHG/keras-multi-head', 'issue_id': 903636350, 'repo_stars': 221.0}