add bfloat16 support for CUDA Neg kernel (#18306)
### Description
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Registers BFloat16 datatype as valid input type for CUDA Neg Kernel.
### Motivation and Context
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- 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>
