Update GQA op schema (#28689)
This pull request makes a targeted update to the operator schema in the
ONNX Runtime codebase, specifically clarifying the optional nature of
certain outputs.
Schema definition improvements:
* Marked the `present_key` and `present_value` outputs as optional in
the `ONNX_MS_OPERATOR_SET_SCHEMA` macro within `bert_defs.cc`, making
the operator schema clearer and more flexible for consumers.
