Add query and candidate

[sahel-sh]

This cl:

• adds rerank_batch, query and candidate data classes.
• removes doc inline demo
• retriever only supports dataset or custom index,
• pyserini_retriever creates requests
• updates all demos to work with rerank_batch
• rerank (non-batch) will be added in a follow up cl
• tests will be updated in a follow up cl
• if we decide to keep pyserini retriever, caching will be reenabled in a followup cl.

Pull Request Checklist

Reference Issue

Please provide the reference to issue this PR is addressing (# followed by the issue number). If there is no associated issue, write "N/A".

ref: #110

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• Have you followed the contributing guidelines?
• Have you verified that there are no existing Pull Requests for the same update/change?
• Have you updated any relevant documentation or added new tests where needed?

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Documentation content changes
• Other...
  • Description:

[ronakice]

Some minor nits but looks good to me besides

[ronakice]

LGTM!