Fix TFSequenceSummary's activation
[TFSequenceSummary](https://github.com/huggingface/transformers/blob/52d2e6f6e904ef9b75c78716ce77b98196ed837a/src/transformers/modeling_tf_utils.py#L1910) set its `activation` only when `config.summary_activation == "tanh"`.

https://github.com/huggingface/transformers/blob/52d2e6f6e904ef9b75c78716ce77b98196ed837a/src/transformers/modeling_tf_utils.py#L1960

However, [ElectraConfig](https://github.com/huggingface/transformers/blob/52d2e6f6e904ef9b75c78716ce77b98196ed837a/src/transformers/models/electra/configuration_electra.py#L137) has default value `"gelu"` for `summary_activation`, and therefore [TFElectraForMultipleChoice](https://github.com/huggingface/transformers/blob/52d2e6f6e904ef9b75c78716ce77b98196ed837a/src/transformers/models/electra/modeling_tf_electra.py#L1427) doesn't use activation at all for `self.sequence_summary`.

This is different from PyTorch version: [SequenceSummary](https://github.com/huggingface/transformers/blob/52d2e6f6e904ef9b75c78716ce77b98196ed837a/src/transformers/modeling_utils.py#L2198)
https://github.com/huggingface/transformers/blob/52d2e6f6e904ef9b75c78716ce77b98196ed837a/src/transformers/modeling_utils.py#L2242
(which has no condition on `tanh`).

This inconsistency also makes the extended pt/tf equivalence fail for `electra` model.

This PR fixes the above issue.


TF: @gante @Rocketknight1

cc @patil-suraj for his somehow related work on adding `ElectraForMultipleChoice` (#4954) with `gelu` for `summary_activation`
_The documentation is not available anymore as the PR was closed or merged._<|||||>LGTM too - @ydshieh are you ready for me to merge this?<|||||>> LGTM too - @ydshieh are you ready for me to merge this?

Yes, go ahead :-) Thanks!