Auto-enable Vulkan in builds, and update docs for Vulkan backend

[Reubend]

Summary

Following up from #20138, this enables the Vulkan backend in source builds when its deps are present, mirroring what we do for CUDA, QNN, and OpenVINO.

It also updates the docs for the Vulkan backend to reflect the latest platform support.

fixes:#20140

Changes

• Added is_vulkan_available(), based on the existing is_cuda_available()
• Vulkan will be enabled in builds when the above returns true and when the submodules are present (making sure that those aren't uninitialized)
• Updated the docs to reflect the fact that the Vulkan backend can now run on Linux and Windows

Testing

I tested these changes on a Linux machine with an A100. I'm relying on CI for Windows testing.

cc @SS-JIA @manuelcandales @digantdesai @cbilgin

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20525

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 1 New Failure, 1 Unrelated Failure, 2 Unclassified Failures

As of commit 5b76a39 with merge base a03f97b ():

NEW FAILURE - The following job has failed:

• pull / unittest-buck / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

UNCLASSIFIED FAILURES - DrCI could not classify the following jobs because the workflow did not run on the merge base. The failures may be pre-existing on trunk or introduced by this PR:

• Build Aarch64 Linux Wheels / pytorch/executorch / build-wheel-py3_10-cpu-aarch64 (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  /__w/executorch/executorch/pytorch/executorch/backends/apple/coreml/runtime/inmemoryfs/inmemory_filesystem.cpp:722:48: error: ‘inmemoryfs::InMemoryFileSystem::InMemoryNode::Kind’ has not been declared
• Build Aarch64 Linux Wheels / pytorch/executorch / upload / upload-wheel-py3_10-cpu-aarch64 (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  Unable to download artifact(s): Artifact not found for name: pytorch_executorch__3.10_cpu_aarch64

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-models-linux (mv2, portable, linux.2xlarge) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128

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[meta-codesync]

@Reubend has imported this pull request. If you are a Meta employee, you can view this in D109868652.