What's the difference between on_fit_start and on_train_start in LightningModule?

[marsggbo]

What's the difference between on_fit_start and on_train_start hooks in LightningModule?

[tshu-w]

I think the document here has answered your question very well.

[awaelchli]

correct, on fit_start is just running much earlier than the train_start, because in between them there is also the pretraining routine that e.g. sanity checks the val loop.