Add Neuron backend

[dacorvo]

What does this PR do?

This adds the neuron backend that was previously maintained in the optimum-neuron repository.

This backend is built on top of the AWS Neuron SDK, and comprises:

• the legacy v2 TGI launcher and router,
• a neuron specific inference server for text-generation.

Documentation

A dedicated documentation page has been added in the backends subsection.

Tests

The backend comes with some dedicated tests:

• neuron server tests (using only the server python package),
• integration tests (using docker images).

I did not add continuous integration yet: waiting for advice on how to do it best.

[HuggingFaceDocBuilderDev]

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