Fixed default config for `Pix2Struct` model to set `Pix2StructTextModel` to `is_decoder=True`
Previously, the `Pix2StructTextModel` was configured with `is_decoder=False` by default causing the attention mask used for self-attention to be non-causal and causing fine-tuning to fail.

As a fix, this PR adds `is_decoder=True` default kwarg to the `Pix2StructTextConfig` class in order to correctly configure the text model as a decoder. 

Fixes # 22903

@younesbelkada
_The documentation is not available anymore as the PR was closed or merged._<|||||>Just updated all pix2struct checkpoints that are under `google` org! Thanks again @gbarello-uipath for flagging this<|||||>Shouldn't the matcha and deplot checkpoints be updated as well?<|||||>Good point, will update them now<|||||>Just updated them! Thanks for flagging @RainbowMan1 