Tests for Spaces requirements.txt upon push of new module

[mathemakitten]

Upon merges we've sometimes had metrics Spaces break because of missing requirements which then need to be added to requirements.txt later.

Not sure what the best way to detect this is, though — sometimes it's not obvious because the requirement is wrapped in a library which has optional dependencies (e.g. this one requires torch because it loads a torch classifier, but you wouldn't know it from the imports).

[lvwerra]

Maybe a bit down the road we could similar to the doc builder create a test org or just a space with the new module and check if the space is up and running. When the PR is merged the test space could be deleted.

Alternatively, we could also setup an env for each space and spin up the gradio app. This is potentially quite slow and the tests are already getting pretty slow.