Add inference throughput benchmark on-prem vllm

[WuhanMonkey]

What does this PR do?

This is the 1st PR as part of the series to add inference throughput benchmarks for Llama 2 models.
In this PR, it adds benchmark scripts, sample input prompts and instructions to run throughput benchmark on-prem for vLLM containers.
The reasons on why we are adding these benchmarks and upcoming benchmarks are in the README file.

Feature/Issue validation/testing

Please describe the tests that you ran to verify your changes and relevant result summary. Provide instructions so it can be reproduced.
Please also list any relevant details for your test configuration.

• Test A
  Logs for Test A

• Test B
  Logs for Test B

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Thanks for contributing 🎉!

[jeffxtang]

Nice benchmark addition to the repo! Just added some text edit suggestions.

[WuhanMonkey]

Addressed comments. Need code testing.

[HamidShojanazeri]

Thanks @WuhanMonkey for the PR, added few comments. I also suggest to rename the folder tobenchmarks/inference/on-prem/vllm instead of inference_throughput.

[bilaalmirza]

Great new benchmark in the repo! I've made some text edits for clarity.

[HamidShojanazeri]

LGTM, please resolve the lint error.