changed readme.md and parameters.json to support llama3 vllm benchmark

[wukaixingxp]

What does this PR do?

To address issue 490, this PR changed readme.md and parameters.json to support llama3 on prem vllm benchmark.

Feature/Issue validation/testing

• [x] benchmark test with 70B instruct

  ~/work/llama-recipes/recipes/benchmarks/inference_throughput/on-prem/vllm (fix/llama3_vllm_benchmark)]$ python chat_vllm_benchmark.py
  Number of token for input prompt: 1000
  | Number of Concurrent Requests | P50 Latency (ms) | P99 Latency (ms) | RPS | Output Tokens per Second | Output Tokens per Second per GPU | Input Tokens per Second | Input Tokens per Second per GPU |Average Output Tokens per Second per Request | Number of Requests Below Threshold |
  |-------------------------------|------------------|------------------|------------------|-------------------|---------------------------|---------------------|------------------------|-------------------------------------- | ---------------------------------- |
  | 1 | 6695.15 | 6695.15 | 0.15 | 50.17 | 6.27 | 149.33 | 18.67 | 50.19 | 0.00 |
  | 2 | 6768.25 | 6768.36 | 0.30 | 94.22 | 11.78 | 295.35 | 36.92 | 47.13 | 0.00 |
  | 4 | 7162.58 | 7163.63 | 0.56 | 184.36 | 23.05 | 558.25 | 69.78 | 46.11 | 0.00 |
  | 8 | 7882.20 | 7886.27 | 1.01 | 327.32 | 40.91 | 1014.16 | 126.77 | 40.95 | 0.00 |
  | 16 | 8446.07 | 8452.66 | 1.89 | 615.40 | 76.93 | 1891.72 | 236.47 | 38.52 | 0.00 |
  | 32 | 11441.68 | 11459.27 | 2.79 | 904.25 | 113.03 | 2791.17 | 348.90 | 28.34 | 0.00 |
  | 64 | 16831.35 | 16875.13 | 3.79 | 1228.25 | 153.53 | 3789.42 | 473.68 | 19.26 | 0.00 |
  | 128 | 27262.07 | 27384.59 | 4.67 | 1515.69 | 189.46 | 4670.50 | 583.81 | 11.90 | 0.00 |
  | 256 | 49327.59 | 49503.46 | 5.17 | 1681.48 | 210.19 | 5167.27 | 645.91 | 6.60 | 255.00 |

Before submitting

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