Allow `extra_epochs` flag in `Trainer.fit` to control finetuning time

🚀 Feature

Trainer(max_epochs=100).fit(model, train_dl, ckpt_path=ckpt_path, extra_epochs=True) would finetune for 100 epochs

Motivation

Finetuning for N epochs requires knowing the previous number of epochs M and setting Trainer(max_epochs=M+N). Google did not tell me how to achieve this.

Pitch

Finetuning training time or number of epochs should be configurable.

Alternatives

Setting many epochs and manually stopping

Additional context

It would be cool with max_time too. I hope this is already solved and this issue is unnecessary.

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cc @justusschock @kaushikb11 @awaelchli @borda @rohitgr7

[carmocca]

You accomplish this by doing:

trainer.fit_loop.max_epochs += 100

before trainer.fit() is called

[carmocca]

There are 2 potential solutions:

1. Pre-load the checkpoint manually

ckpt = torch.load(...)
current_epoch = ckpt["current_epoch"]
trainer = Trainer(max_epochs=current_epoch + N)

An issue with this method is that it loads the fully checkpoint just for this change. This relates to #5339 and https://github.com/Lightning-AI/lightning/issues/12712

1. Extract the state from the checkpoint in on_load_checkpoint and modify the Trainer’s max_epochs. This requires editing the LightningModule hook to do this or creating a Callback just for it.