Fix parity checker in LLaMA scripts (#20301)
### Description
This PR fixes the parity checker in the LLaMA scripts by adding the
following.
- Enable buffer sharing manually with `use_buffer_share` instead of
`use_gqa`
- Get max sequence length from model's config
### Motivation and Context
This PR fixes an issue with running the parity checker on other
large-language models where `GroupQueryAttention` can be used without
buffer sharing enabled.
