LayerNorm test fix (#13840)
### Description
<!-- Describe your changes. -->
Testcases of LayerNorm with fp16/bf16 are failed in Andriod and IOS
since the two platforms don't support the combinations of datatypes as
well.
https://dev.azure.com/onnxruntime/onnxruntime/_build?definitionId=134&_a=summary
https://dev.azure.com/onnxruntime/onnxruntime/_build?definitionId=53&_a=summary
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
