[Diffusers CI] Switch slow test runners

[DN6]

What does this PR do?

Moves the diffusers slow tests from the V100 runners to the pool of T4 runners.

The timeout issues in the diffusers slow test CI is likely coming from a lack of RAM. The T4 runners are g4dn.4xlarge instances with 64GB RAM, which should help mitigate the issue. Additionally, there are more T4 runners available in the pool than V100s (2 V100s vs 40+ T4s), which means that more of our pipelines tests can run in parallel. So we should hopefully see a speed up in the CI as well.

Since we're switching GPUs some precision related failures are expected. I will open PRs to follow up on those issues as they occur.

Fixes # (issue)

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