Gemma: update activation warning
Author: pcuenca
Pull request: yes

# What does this PR do?

This is a nit PR, but I was confused. I got the warning even after I had changed `hidden_act` to `gelu_pytorch_tanh`, telling me that I was using the "legacy" `gelu_pytorch_tanh`.

Another option is to keep the warning but change the message to say something like "`hidden_act` is ignored, please use `hidden_activation` instead. Setting Gemma's activation function to `gelu_pytorch_tanh`".

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Comments:
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Yeah, I get it, but the message I got was:

> Gemma's activation function should be approximate GeLU and not exact GeLU.
Changing the activation function to `gelu_pytorch_tanh`.if you want to use the legacy `gelu_pytorch_tanh`, edit the `model.config` to set `hidden_activation=gelu_pytorch_tanh`   instead of `hidden_act`. See https://github.com/huggingface/transformers/pull/29402 for more details.

> edit the model.config to set hidden_activation=gelu_pytorch_tanh instead of hidden_act.

Sounds alright to me no? Feel free to make it clearer! 

But

> if you want to use the legacy gelu_pytorch_tanh, edit the model.config to set hidden_activation=gelu_pytorch_tanh

I'm not using a legacy function, and the warning says it's changing it when it isn't. So my suggestion was to either hide the warning (only in this case, when the activation matches what we want), or if we want to encourage the use of `hidden_activation`, just say that.

You are right, let's encourage the use of `hidden_activation` . Note that is is changing it: 
```python 
        if config.hidden_activation is None:
            logger.warning_once(
                "Gemma's activation function should be approximate GeLU and not exact GeLU.\n"
                "Changing the activation function to `gelu_pytorch_tanh`."
                f"if you want to use the legacy `{config.hidden_act}`, "
                f"edit the `model.config` to set `hidden_activation={config.hidden_act}` "
                "  instead of `hidden_act`. See https://github.com/huggingface/transformers/pull/29402 for more details."
            )
            hidden_activation = "gelu_pytorch_tanh"
        else:
            hidden_activation = config.hidden_activation
        self.act_fn = ACT2FN[hidden_activation]
```
the `hidden_activation = "gelu_pytorch_tanh"` is used. But it's not changing `config.hidden_activation` which is also something we can do!  

Yes, `hidden_activation = "gelu_pytorch_tanh"`, I meant that it's no different to what the user expressed with `config.hidden_act = "gelu_pytorch_tanh"` :) I'll submit a wording change, I didn't think about changing the `config` too, we can do that if you think it helps :)

Forgot about this, merging now.