[wip-prototype]chunked prefill on 1k prompt len

[mailvijayasingh]

Description

PR is a dirty first round of PR for chunked prefill based
https://arxiv.org/abs/2308.16369

Attention masks are calculated based on
https://medium.com/byte-sized-ai/llm-inference-optimizations-2-chunked-prefill-764407b3a67a

The rest of the description includes relevant details and context, examples:

• why is this change being made,
• the problem being solved and any relevant context,
• why this is a good solution,
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• shortcomings of the solution and possible future improvements.

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FIXES: b/123456
FIXES: #123456

Tests

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Checklist

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• I have performed a self-review of my code.
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• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.