[QNN] Relax tolerances for ONNX attention node tests (#26004)

### Description
* Relax tolerances for ONNX attention node tests

### Motivation and Context
* Tests failing in Qualcomm Internal CI before relaxing tolerances, tests passing after

Author: qti-kromero

onnxruntime/test/testdata/onnx_backend_test_series_overrides.jsonc

@@ -4,6 +4,8 @@
     // Key: str, the name of the test as defined by ONNX without any device suffix.
     // Val: float, max absolute difference between expected and actual.
     "atol_overrides": {
+        "test_attention_4d_fp16": 5e-4,
+        "test_attention_4d_gqa_with_past_and_present_fp16": 6e-4,
         "test_dft": 1e-3,
         "test_dft_axis": 1e-3,
         "test_dft_inverse": 1e-3,