docs: Update nightly package install instructions

[fs-eire]

Description

The existing nightly install command uses --index-url, which makes the PyTorch nightly index the only package source. This can fail in clean environments when Torch-TensorRT dependencies must be resolved from PyPI. Adding --extra-index-url https://pypi.org/simple keeps the PyTorch nightly index available for Torch/PyTorch nightly wheels while allowing pip to resolve PyPI-hosted dependencies.

Type of change

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Checklist:

• My code follows the style guidelines of this project (You can use the linters)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas and hacks
• I have made corresponding changes to the documentation
• I have added tests to verify my fix or my feature
• New and existing unit tests pass locally with my changes
• I have added the relevant labels to my PR in so that relevant reviewers are notified

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