feat: expose GPU energy consumption (mJ) in responses

[JulienDelavande]

What does this PR do?

This PR adds energy consumption measurement to each request served by the router.
A new field energy_mj (in millijoules) is added to the details section of the response.

Example usage:

curl 127.0.0.1:3000/generate \
    -X POST \
    -d '{"inputs":"What is Deep Learning?","parameters":{"max_new_tokens":20, "details":true}}' \
    -H 'Content-Type: application/json'

Response (excerpt):

{
  "generated_text": "Deep learning is a subset of machine learning...",
  "details": {
    "finish_reason": "length",
    "generated_tokens": 20,
    "tokens": [...],
    "energy_mj": 108836
  }
}

Motivation

• Provide users with direct insight into the energy usage of their requests.
• Enable frontends (e.g. [Chat UI Energy Demo](https://huggingface.co/spaces/jdelavande/chat-ui-energy)) to display this metric alongside generated text.
• Support sustainability efforts by making energy costs more transparent.

Limitations

• The measurement is taken at the GPU level during the full processing of the request by the router.
• Energy is not attributed per user when multiple requests are batched — values represent an approximation of total GPU energy consumption.
• Despite this, the metric remains useful to give users an idea of the energy footprint of their queries without deploying TGI themselves.

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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](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
  Pull Request section?
• Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
  [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
• Did you write any new necessary tests?

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Who can review?

@regisss @Narsil

[regisss]

LGTM!