CI: What is the purpose of `slow.yml`

### 🐛 Describe the bug

What is the purpose of `slow.yml` job, when we can shard more and probably can rely on TD to skip slow tests if they are not needed?

In the past `slow.yml` job was a way of keeping time to signal low, while running some tests post commit, but now that we have TD we probably can get rid of concept of slow test and just run them conditionally on TD's decision

At the very least, I think this job should be viable/strict blocking, as it just runs a subset of tests from pull request, that are decorated with `@slowTest` or take more than 90 sec to finish
### Versions

CI

cc @seemethere @pytorch/pytorch-dev-infra