* Run AMP tests in their own process
With opt_level="O1" (the default), AMP patches many
torch functions, which breaks any tests that run afterwards.
This patch introduces a pytest extension that lets
tests be marked with @pytest.mark.spawn so that they
are run in their own process using torch.multiprocessing.spawn
so that the main python interpreter stays un-patched.
Note that tests using DDP already run AMP in its own process,
so they don't need this annotation.
* Fix AMP tests
Since AMP defaults to O1 now, DP tests no longer throw exceptions.
Since AMP patches torch functions, CPU inference no longer works.
Skip prediction step for AMP tests.
* typo
Nic Eggert