fix LayerNorm f16 CPU implementation (#22479)
### Description
The recent PR #22223 introduced 2 bugs in implementation of CPU
LayerNorm f16:
- possible access to nullptr for bias
`const TensorShape& bias_shape = bias->Shape();` will crash when `bias`
does not exist. (amazingly seems this one is not coverred by any test
case)
- fix: guard with pointer check
- a racing condition inside ComputeJob
`ComputeJob()` is dispatched to threadpool and it internally tries to
modify `LayerNormImpl::scale_fp32_` and `LayerNormImpl::bias_fp32_`,
which are `std::unique_ptr`s and are not thread-safe.
- fix: move the modification of `LayerNormImpl::scale_fp32_` and
`LayerNormImpl::bias_fp32_` out of `ComputeJob()` and put into
`LayerNormImpl::ComputeWithoutContext()`. It may still have racing
condition because `ConcurrentRunSupported` is set to `true` for CPU EP.
Added an OrtMutex.
This should fixes the recent flaky tests as well.
