onnxruntime

add bfloat16 support for where operator (#18118)

### Description
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Adds bfloat16 as a valid input parameter type for where node for ONNX
opset 16+.

### 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. -->

Enabling `meta-llama/Llama-2-70b` to be finetuned with ONNX Runtime
training.

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Co-authored-by: Prathik Rao <prathikrao@microsoft.com@orttrainingdev8.d32nl1ml4oruzj4qz3bqlggovf.px.internal.cloudapp.net>