[QNN EP] Fix GatherND int64 unique name creation (#26014)
- Making sure that the Uniquename function is not called repeatedly
- Added unit test for int64 GatherND test for coverage
### Description
GatherND op is generating a unique name twice which is causing an issue
when Casting int32 tensor back to int64 for output.
### Motivation and Context
Making sure that the UniqueName function is just called once and reusing it for creating the CastOp.
