Remove requirement to be in training mode to capture activations

[lucmos]

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Docs change / refactoring / dependency upgrade

Motivation and Context / Related issue

In some contexts it is necessary to compute gradients during validations and/or testing.

However, there is currently an explicit check that -- even if the gradients are manually enabled -- activations are captured solely during training.

This leads to the folllowing error:

How Has This Been Tested (if it applies)

Manual testing on local project.

Checklist

• [x] The documentation is up-to-date with the changes I made.
• [x] I have read the CONTRIBUTING document and completed the CLA (see CONTRIBUTING).
• [ ] All tests passed, and additional code has been covered with new tests.

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[HuanyuZhang]

Thanks for the contribution to Opacus. Just one question, it is not clear to me when per-sample gradient is needed beyond training. Could you provide some examples? Thanks!

[lucmos]

Thanks for you work on Opacus! 🍻

I think they may be needed whenever the gradients are enabled. Why would one need batch-gradients but not sample-gradients?

As a specific example atm I can only provide the research project I am working on, it's about a completely different topic (out of distribution generalization) -- still I think Opacus could be useful beyond it's original scope in differential privacy 🙂

[HuanyuZhang]

I am still hesitant to make the changes since it may confuse people. Please make custom changes to suit your needs. Thx!