Main, knn_classifier and simclr modifications (#1901)

* Main, knn_classifier and simclr modifications

* Fixed knn_classifier to not break with other models + removed redundant forward pass

* Addressing reviews

* fix minor things

* formatting

* fix unit tests

* fix unit tests

* fix unit tests

* fix knn eval keys in vitb16 benchmarks

* unit tests

* only log knn if LightningModule has a Trainer attached

* formatting

* remove val_check_interval

* apply normalization even if model does not exist

* import future annotations

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Co-authored-by: liopeer <lionel@lightly.ai>