Adapt find_unused_parameters warning

[carmocca]

🚀 Feature

Latter PyTorch versions introduced the following warning

https://github.com/pytorch/pytorch/blob/d0adb5ff264df8e0e057ce1178feb30198c601d2/torch/csrc/distributed/c10d/reducer.cpp#L1256-L1264

which appears when find_unused_parameters=True but it's not necessary.

Motivation

We set find_unused_parameters=True by default, so this warning will appear for a large percentage of users.

Pitch

Catch this warning and produce another one that explicitly mentions how to modify the strategy passed.

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cc @Borda @akihironitta

[carmocca]

This is blocked by pytorch/pytorch#75725