guard on model.eval when using torch.compile + FSDP2
Author: winglian
Pull request: yes

# What does this PR do?
If the model is torch.compile(d) and if we're using FSDP2, after `self._wrap_model(...)` is called, the model is actually the `model.forward` function, so it's not possible to call `.eval()` on it


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Comments:
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LGTM - I'm happy for @sunmarc to merge if he's happy with it!

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