feat: Add automatic nightly benchmarks

[Hugoch]

What does this PR do?

This PR adds automated load tests in CI using custom benchmarkedtool.

Tests are performed using a Constant arrival rate load test: It simulates a constant rate of user requests arrival, independent of the system’s response rate, during 120 seconds.

The test is run for using a sample of ShareGPT.

Test compute the following metrics:

• Inter token latency: Time to generate a new output token for each user querying the system. It translates as the “speed” perceived by the end-user. We aim for at least 300 words per minute (average reading speed), so ITL<150ms
• Time to First Token: Time the user has to wait before seeing the first token of its answer. Lower waiting time are essential for real-time interactions, less so for offline workloads.
• End-to-end latency: The overall time the system took to generate the full response to the user.
• Throughput: The number of tokens per second the system can generate across all requests
  Successful requests: The number of requests the system was able to honor in the benchmark timeframe
• Error rate: The percentage of requests that ended up in error, as the system could not process them in time or failed to process them.

At the end of the test, it produces a dashboard with the results and plots.

Results are added to https://huggingface.co/spaces/huggingface/tgi-benchmarks

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

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Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
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