onnxruntime

refactor some code (#15566)

### Description
<!-- Describe your changes. -->

1. moved onnxruntime/contrib_ops/cuda/decoder to
onnxruntime/contrib_ops/cuda/bert
2. create utils.cuh under /bert for shared implementations in
decoder_masked_multihead_attention_impl_utils.h and
rotary_embedding_util.h
3. refactored relative_attn_bias_impl.cu by reusing the template
specializations in utils.cuh

### 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. -->

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Co-authored-by: Ubuntu <wy@v100-2.0cdb2e52twzevn1i4fi45bylyg.jx.internal.cloudapp.net>