Gemma: update activation warning (#29995)
* Gemma: only display act. warning when necessary
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`".
* Change message, and set `config.hidden_activation`
