fix progress bar restart with fault-tolerant training enabled

[awaelchli]

What does this PR do?

Fixes a TODO in #9130

The progress bar callback did not resume correctly, because of two reasons:

• on_train_start() was resuming the correct progress value but on_train_epoch_start() would overwrite it back to 0
• the on_*_epoch_start hooks reset the tqdm counter to 0 instead of the value we would resume from

This came up in the context of fault tolerant training.

Repro script

import os
from time import sleep

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

from pytorch_lightning import LightningModule, Trainer


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):
        sleep(0.5)
        print(batch_idx, self.trainer.fit_loop.batch_idx)
        loss = self(batch).sum()
        self.log("train_loss", loss)
        return {"loss": loss}

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


def run():
    train_data = DataLoader(RandomDataset(32, 64), batch_size=2)

    model = BoringModel()
    trainer = Trainer(
        default_root_dir=os.getcwd(),
        weights_summary=None,
        # resume_from_checkpoint=".pl_auto_save.ckpt",
    )
    trainer.fit(model, train_dataloaders=train_data)


if __name__ == "__main__":
    run()

The added test fails on master.

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[codecov]

Codecov Report

Merging #9310 (f97953b) into master (f6d4087) will increase coverage by 0%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #9310    +/-   ##
=======================================
  Coverage      92%     92%            
=======================================
  Files         178     178            
  Lines       14850   15402   +552     
=======================================
+ Hits        13701   14241   +540     
- Misses       1149    1161    +12     

[tchaton]

Neat !