[Bug Report] ActivationCache.to() always warns "move_model is deprecated", even when not passed

[RecreationalMath]

Describe the bug

ActivationCache.to(device) emits DeprecationWarning("The 'move_model' parameter is deprecated.") on every call, even when the caller does not pass move_model. The guard at ActivationCache.py:208 is if move_model is not None:, but the parameter defaults to False, and False is not None, so the warning fires unconditionally. The normal cache-offload idiom cache.to("cpu") therefore prints a noisy deprecation warning.

Relatedly, the v3.0 release notes state this parameter was already removed: "Two deprecations announced in 2.0 have now been removed - the move_model parameter in ActivationCache.to..." (release-3.0.md). But it is still present in the code on main (v3.2.1) and dev, so this is also a stale-deprecation issue.

Code example

import warnings
from transformer_lens.model_bridge import TransformerBridge
model = TransformerBridge.boot_transformers("distilgpt2", device="cpu")
_, cache = model.run_with_cache(model.to_tokens("hello world"))
with warnings.catch_warnings(record=True) as w:
    warnings.simplefilter("always")
    cache.to("cpu")
print([str(x.message) for x in w])
# -> ["The 'move_model' parameter is deprecated."]

The warning is also visible under Python's default filter (no simplefilter needed): a plain cache.to("cpu") prints ActivationCache.py:209: DeprecationWarning: The 'move_model' parameter is deprecated. to stderr.

System Info

• Installed from source (main @ v3.2.1; dev also affected), uv
• macOS arm64 (also: any OS, this is a pure-Python guard bug)
• Python 3.12

Additional context

Found while implementing generate(return_cache=True) (#697), the #1337's device-offload path writes cache_dict directly rather than calling cache.to, in part to avoid this warning. The buggy guard was introduced in #432 and has shipped in every tag from v1.10.0 through v3.2.1.

Open question for the maintainer: given the v3.0 release notes already announced removal of move_model, shouldn't this parameter simply be removed (and the warning along with it), or is there a deliberate reason to keep the deprecated path?

Checklist

• I have checked that there is no similar issue in the repo

[RecreationalMath]

Having thought some more about it, I lean towards just removing the move_model parameter and the warning with it. It has been (correctly) deprecation-warned on the truthy path since v2.0 and incorrectly warned on every call since v1.10.0 (~2.5 years), so any user still on the deprecated path has had ample notice on top of what the v3.0 notes already announced.

Happy to PR this. I'm thinking two PRs, one each for main (since the bug ships in v3.2.1) and dev (so it does not get re-shipped in the next release). Sound good? @jlarson4

[jlarson4]

@RecreationalMath Feel free to tackle this as well. I agree with just removing the parameter and the warning. Strange that it seems to have snuck its way back into the code after being removed. Please PR specifically to dev, since this is not a breaking issue it can wait for the next release.