Make ViTPooler configurable (#36517)
* Make ViT Pooler configurable, so that it is possible to pick the activation function and the number of channels in the output
* Add documentation and allow functions as activations (instead of just string)
* formatting change
* Use ACT2FN
* Formatting change
* Formatting changes
* force pooler_act to be string
* force pooler_act to be string
* Add configs to OBJECTS_TO_IGNORE to make check_docstrings happy
* Making the same change in ijepa to make check_modular_conversion happy
* Add IJepaConfig to make CI happy
* rename pooler_size to pooler_output_size as defined in the config
* typo
* revert change to ignore variable
* Ran utils/check_docstrings.py --fix_and_overwrite
* revert unrelated change
* remove redundant defaults
* rename self.act -> self.activation
* tanh activation function in mapping
