Find another way to disable grad in tests

[lkct]

Ref:

• Make running tests reliable and faster #87
• Disable grads and enable determinism in tests #126

I'm now against the solution to #87 by #126 on no_grad, as partially reverted here.

Reason: globally disabling grad is counter-intuitive and can waste time in debugging tests to look for missing grad, especially now when we have differential functionals.

Alternative: if a model does not need grad in unit tests, we can use nn.Module.requires_grad_(False) to disable grad for this module's params.

[loreloc]

I think more than 90% of tests will not require computing gradients. So, I still think a good default setting is gradients in tests turned off. Keeping this for now as we have still to write many tests, but it might be closed.

[lkct]

Just to note that I encountered this again 😢, though it quickly came to me to check if gradient is enabled (because it's not the first time).

Perhaps even if we're keeping the behaviour, we should add smth to docs/faqs/dev notes to remind people.

PS: currently we lack tests for circuit training which also needs grad

[andreasgrv]

Maybe can use a py.test fixture for this:

https://docs.pytest.org/en/6.2.x/fixture.html