Fix require_backends_available to reenable distributed tests

[H-Huang]

Stack from ghstack (oldest at bottom):

• -> Fix require_backends_available to reenable distributed tests #101704

TLDR

Fix decorator to re-enable 26+ distributed tests that were previously being skipped in CI

Explanation

As part of the UCC upstream, we updated the backend tests cases to also include "ucc".

pytorch/torch/testing/_internal/common_distributed.py

Lines 90 to 92 in 3ed1569

	backend_feature["gpu"] = {"nccl", "gloo", "ucc"}
	backend_feature["cuda"] = {"nccl", "gloo", "ucc"}
	backend_feature["ddp"] = {"nccl", "gloo", "ucc"}

In distributed tests we use a decorator which reads from this config and makes sure all backends are available on the system.

pytorch/torch/testing/_internal/distributed/distributed_test.py

Line 7131 in 3ed1569

@require_backends_available(DistTestCases.backend_feature["gpu"])

However, UCC is not configured on by default for a certain subset of CI tests, which causes the entire test to be skipped (even if the test is meant for nccl and the backend being tested is nccl).

As the fix, we should just check that only the BACKEND being tested is available

Changes

• Change logic to only check if the current backend being used is available
• Rename require_backends_available -> require_backend_is_available

[pytorch-bot]

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[rohan-varma]

great catch~

[H-Huang]

@pytorchbot merge

[pytorchmergebot]

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