Missing `f` prefix on f-strings

[code-review-doctor]

Some strings looks like they're meant to be f-strings but are missing the f prefix meaning variable interpolation won't happen.

https://github.com/PyTorchLightning/pytorch-lightning/blob/9b2b1bb494f928137be67e325b3fc8544a3bf321/pytorch_lightning/utilities/enums.py#L144
https://github.com/PyTorchLightning/pytorch-lightning/blob/9b2b1bb494f928137be67e325b3fc8544a3bf321/pytorch_lightning/utilities/enums.py#L164

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