Some changes to Sampling Op (#14218)
### Description
<!-- Describe your changes. -->
1. add an optional input to pass in seed
2. two UTs. one for top_p=0.5, another for top_p=0.01(create greedy
search result, in convert_generation.py)
3. fix a bug in cpu kernel
### 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. -->
Co-authored-by: Ubuntu <wy@v100-2.0cdb2e52twzevn1i4fi45bylyg.jx.internal.cloudapp.net>
