Fix FlaxRoFormerClassificationHead activation
# What does this PR do?

Current `FlaxRoFormerClassificationHead` use hard-coded activation `nn.tanh`, but the PyTorch version `RoFormerClassificationHead` use `ACT2FN[self.config.hidden_act]` which is by default `gelu` (and this is the one used by the default checkpoint's config).

This PR fixes this activation issue in `FlaxRoFormerClassificationHead`.
To get more familiar with the library management/maintenance, I would like hear from you regarding: does this count as a breaking change?
<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>Merge now since this is clearly a bug, as well as a tiny change