Resuming training resets the logged step number

🐛 Bug

The change introduced in https://github.com/PyTorchLightning/pytorch-lightning/pull/11805 causes a reset to the logged step number. https://github.com/PyTorchLightning/pytorch-lightning/blob/49a4a36ad45b937dd0124ecfb08eb7400dbf3950/pytorch_lightning/trainer/connectors/logger_connector/logger_connector.py#L122

To Reproduce

import os

import torch
from torch.utils.data import DataLoader, Dataset

from pytorch_lightning import LightningModule, Trainer
from pytorch_lightning.callbacks import ModelCheckpoint


class RandomDataset(Dataset):
    def __init__(self, size, length):
        self.len = length
        self.data = torch.randn(length, size)

    def __getitem__(self, index):
        return self.data[index]

    def __len__(self):
        return self.len


class BoringModel(LightningModule):
    def __init__(self):
        super().__init__()
        self.layer = torch.nn.Linear(32, 2)

    def forward(self, x):
        return self.layer(x)

    def training_step(self, batch, batch_idx):
        loss = self(batch).sum()
        self.log("train_loss", loss)
        return {"loss": loss}

    def validation_step(self, batch, batch_idx):
        loss = self(batch).sum()
        self.log("valid_loss", loss)

    def configure_optimizers(self):
        return torch.optim.SGD(self.layer.parameters(), lr=0.1)


def run(ckpt_path=None):
    train_data = DataLoader(RandomDataset(32, 64), batch_size=2)
    val_data = DataLoader(RandomDataset(32, 64), batch_size=2)

    model = BoringModel()
    trainer = Trainer(
        default_root_dir=os.getcwd(),
        num_sanity_val_steps=0,
        max_epochs=2,
        enable_model_summary=False,
        callbacks=ModelCheckpoint(dirpath="checkpoints", save_top_k=-1, filename="{epoch}", save_on_train_epoch_end=False),
        log_every_n_steps=1,
    )
    trainer.fit(model, train_dataloaders=train_data, val_dataloaders=val_data, ckpt_path=ckpt_path)


if __name__ == "__main__":
    run()
    run("checkpoints/epoch=0.ckpt")

The script will create two tensorboard logs:

• version_0: steps 0 to 63
• version_1: steps 0 to 31

Expected behavior

• version_1: steps 31 to 63

This was the behavior before https://github.com/PyTorchLightning/pytorch-lightning/pull/11805

Environment

• PyTorch Lightning Version (e.g., 1.5.0): master (49a4a36)
• Fault-tolerant training is off (PL_FAULT_TOLERANT_TRAINING=0)

cc @tchaton @rohitgr7 @akihironitta @awaelchli @ananthsub @ninginthecloud @carmocca

[rbregier]

Hi, this workaround seems to work for my use case:

checkpoint = torch.load(args.ckpt_path, map_location='cpu')
global_step_offset = checkpoint["global_step"]
trainer.fit_loop.epoch_loop._batches_that_stepped = global_step_offset
del checkpoint    
trainer.fit(experiment, datamodule=datamodule, ckpt_path=args.ckpt_path)

[rohitgr7]

cc: @carmocca wdyt?