Update README amd therock install instructions

[alexheretic]

ROCm/TheRock builds have changed.

See ROCm/TheRock RELEASES.

Related: ROCm/TheRock#5954 (comment)

[coderabbitai]

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• README.md

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📝 Walkthrough

Walkthrough

The AMD GPU experimental section of README.md (lines 254–267) is updated to replace the previous RDNA 3/3.5/4-specific installation guidance. The old instructions provided separate pip commands with per-architecture v2/… index URLs. The new instructions use the ROCm multi-arch nightlies index and torch[...] device specifiers, with an explicit example targeting gfx1100 (e.g., 7900 XTX / 7900 GRE) and a second command to install support for all supported devices.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: updating README AMD/TheRock installation instructions, which directly corresponds to the changeset.
Description check	✅ Passed	The description is directly related to the changeset, explaining that ROCm/TheRock builds have changed and providing references to official documentation and related issues.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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