fix(mlflow): Enabling multiple callbacks for checkpoint reporting

[HarryAnkers]

What does this PR do?

Fixes #20584

Currently if the below code is run it will only ever save one checkpoint if both use callbacks and both use param save_top_k. This works if log_model='all' but not when log_model=True. If you flip the order of callbacks this works fine however. I have tried to raise a fix for this. Let me know what you think.

import os
import pytorch_lightning as pl
from pytorch_lightning.loggers import MLFlowLogger
from pytorch_lightning.callbacks import ModelCheckpoint
import torch
import torch.nn as nn
import torch.optim as optim
from torch.utils.data import DataLoader, TensorDataset

# Change these as wanted
os.environ["MLFLOW_TRACKING_USERNAME"] = ""
os.environ["MLFLOW_TRACKING_PASSWORD"] = ""


class SimpleModel(pl.LightningModule):
    def __init__(self):
        super().__init__()
        self.fc = nn.Linear(10, 1)

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

    def training_step(self, batch, batch_idx):
        x, y = batch
        y_hat = self(x)
        loss = nn.MSELoss()(y_hat, y)
        self.log("train_loss", loss)
        return loss

    def validation_step(self, batch, batch_idx):
        x, y = batch
        y_hat = self(x)
        loss = nn.MSELoss()(y_hat, y)
        self.log("val_loss", loss)

    def configure_optimizers(self):
        return optim.SGD(self.parameters(), lr=0.01)


def train():
    URI = ""

    mlflow_logger = MLFlowLogger(
        experiment_name="harry-test",
        tracking_uri=URI,
        run_name="harry-test1",
        log_model=True,
    )

    x_train, y_train = torch.randn(100, 10), torch.randn(100, 1)
    train_dataset = TensorDataset(x_train, y_train)
    train_loader = DataLoader(train_dataset, batch_size=32)

    x_val, y_val = torch.randn(20, 10), torch.randn(20, 1)
    val_dataset = TensorDataset(x_val, y_val)
    val_loader = DataLoader(val_dataset, batch_size=32)

    model = SimpleModel()

    checkpoint_callback_val = ModelCheckpoint(
        dirpath="/home/harryankers/Documents/Checkpoints/val_checkpoints",
        monitor="val_loss",
        filename="best_val_model-{epoch:02d}-{val_loss:.2f}",
        save_top_k=2,
        mode="min",
    )

    checkpoint_callback_train = ModelCheckpoint(
        dirpath="/home/harryankers/Documents/Checkpoints/train_checkpoints",
        monitor="train_loss",
        filename="best_train_model-{epoch:02d}-{train_loss:.2f}",
        save_top_k=2,
        mode="min",
    )

    trainer = pl.Trainer(
        max_epochs=20,
        logger=mlflow_logger,
        callbacks=[checkpoint_callback_train, checkpoint_callback_val],
        val_check_interval=3,
    )

    trainer.fit(model, train_loader, val_loader)


if __name__ == "__main__":
    train()

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---

📚 Documentation preview 📚: https://pytorch-lightning--20585.org.readthedocs.build/en/20585/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79%. Comparing base (ea59e40) to head (51da02e).
Report is 1 commits behind head on master.

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HEAD has 2247 uploads less than BASE

Flag	BASE (ea59e40)	HEAD (51da02e)
cpu	525	24
lightning_fabric	67	0
pytest	266	0
python3.9	132	6
lightning	394	18
python3.10	66	3
python3.11	132	6
python3.12.7	195	9
gpu	2	0
pytorch2.1	99	9
pytest-full	261	24
pytorch2.2.2	33	3
pytorch_lightning	66	6
pytorch2.3	33	3
pytorch2.4.1	31	3
pytorch2.5.1	65	6

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20585     +/-   ##
=========================================
- Coverage      88%      79%     -9%     
=========================================
  Files         267      264      -3     
  Lines       23380    23326     -54     
=========================================
- Hits        20481    18367   -2114     
- Misses       2899     4959   +2060     

[Borda]

@HarryAnkers could you pls update the tests accordingly?

FAILED loggers/test_mlflow.py::test_mlflow_log_model[True] - AssertionError: Expected 'log_artifact' to have been called once. Called 0 times.
FAILED loggers/test_mlflow.py::test_mlflow_multiple_checkpoints_top_k - AssertionError: MLFlow did not log any train checkpoint artifacts
assert 0 > 0
 +  where 0 = len([])

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