[Feature Request] expose `unidirectional` (causal) attribute of GQA

[hann-wang]

Describe the feature request

We have a unidirectional attribute in MHA, but it is missing in GQA because most LLMs are causal. Transformer-based Text-to-Image models, on the other hand, are not causal. We should expose this attribute in GQA to help facilitate the deployment of Text-to-Image models.

Describe scenario use case

Text-to-Image generation models (like Stable Diffusion 3) require attention implemented with causal=False.

PR

#23412