Add pytest fixture to limit max time a test can take

[deependujha]

🐛 Bug

To Reproduce

Steps to reproduce the behavior...

Code sample

# Ideally attach a minimal code sample to reproduce the decried issue.
# Minimal means having the shortest code but still preserving the bug.

Expected behavior

If a test hangs that doesn't have a timeout limit, it'll hang forever. PR #473 was very slow (probably hanging and not progressing). With an autouse fixture, we can set a max timeout of 10 minute or something appropriate.

Additional context

Environment detail

• PyTorch Version (e.g., 1.0):
• OS (e.g., Linux):
• How you installed PyTorch (conda, pip, source):
• Build command you used (if compiling from source):
• Python version:
• CUDA/cuDNN version:
• GPU models and configuration:
• Any other relevant information:

[deependujha]

my bad. The test passed, but still, should we have a default max time limit?

https://stackoverflow.com/questions/43598985/how-can-i-set-a-default-per-test-timeout-in-pytest

cc: @tchaton @bhimrazy

[bhimrazy]

For a global max limit, 10 to 20 minutes seems reasonable to me.
And when needed, we can apply the timeout at the test level for specific functions using @pytest.mark.timeout(...).