update d_kv'annotation in mt5'configuration
Author: callanwu
Pull request: yes

# What does this PR do?

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Fixes # (issue)


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## Who can review?
@stevhliu and @MKhalusova

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Comments:
Thank you for your contribution! Please make sure that the quality checks pass - here's the [contributing guide](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md). It looks like you need to run `make style`.

> Thank you for your contribution! Please make sure that the quality checks pass - here's the [contributing guide](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md). It looks like you need to run `make style`.

@MKhalusova Hi, thx for your reminder. I have passed all checks!

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_27585). All of your documentation changes will be reflected on that endpoint.

Add reference:
https://huggingface.co/google/mt5-small/blob/main/config.json
modules output of a loaded mt5-small
```txt
T5Block(
    (layer): ModuleList(
        (0): T5LayerSelfAttention(
        (SelfAttention): T5Attention(
            (q): Linear(in_features=512, out_features=384, bias=False)
            (k): Linear(in_features=512, out_features=384, bias=False)
            (v): Linear(in_features=512, out_features=384, bias=False)
            (o): Linear(in_features=384, out_features=512, bias=False)
        )
        (layer_norm): T5LayerNorm()
        (dropout): Dropout(p=0.1, inplace=False)
        )
        (1): T5LayerFF(
        (DenseReluDense): T5DenseGatedGeluDense(
            (wi_0): Linear(in_features=512, out_features=1024, bias=False)
            (wi_1): Linear(in_features=512, out_features=1024, bias=False)
            (wo): Linear(in_features=1024, out_features=512, bias=False)
            (dropout): Dropout(p=0.1, inplace=False)
        )
        (layer_norm): T5LayerNorm()
        (dropout): Dropout(p=0.1, inplace=False)
        )
    )
)
```

@MKhalusova @patrickvonplaten Can it be merged now? Are there any further improvements needed:)

Thanks @callanwu 🤗