Fix instantiate side effect modifying input config parent

[jesszzzz]

Motivation

See #3001 for details, tldr if you call hydra.utils.instantiate on a subconfig, it might modify the parent config through omegaconf resolution which is an unexpected side effect. While there is currently logic to deepcopy the subconfig to avoid such a side effect, we actually need to deepcopy the entire config to avoid it.

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Yes

Test Plan

New and existing tests pass

Related Issues and PRs

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Fixes #3001

[andywag]

I've been looking in to ways to remove these type of copy operations because they have a huge impact on performance.

Is this copy done only at the top level or is it done recursively?
When does this occur? Only when the config is resolved (ie string interpolation)?
How big of a problem is this?

We should probably check the performance hit for key users before landing it.

[jesszzzz]

Agree it's not ideal for perf but not sure how to get around it given OmegaConf.resolve modifies the config in place

Is this copy done only at the top level or is it done recursively?
The copy only happens at the top level so it should happen just once per user call to instantiate
When does this occur? Only when the config is resolved (ie string interpolation)? How big of a problem is this?
It only happens when the hydra.utils.instantiate function is called, so it's specific to using _target_. It only happens in specific cases so not that common, but very confusing when it happens. Maybe I can look into only doing the deep copy when it's necessary or add a flag to disable it at the user's risk?

We should probably check the performance hit for key users before landing it.
Makes sense, will look more into which key users use this and when!

[andywag]

I'm slightly concerned about the perf but if it only happens once it shouldn't be a big problem.