[QNN-EP] Enable einsum with QK equations for QNN. (#25861)
### Description
Enable einsum op with QK equations for attention in QNN EP.
### Motivation and Context
Current einsum op in QNN doesn't support equations with capital alphabets. Loose this constraint to allow more usecases.
Signed-off-by: Mu-Chein Hsu <quic_muchhsu@quicinc.com>
