fix model_checkpoint.py
[🤖]: Hi @ethanhe42 👋,

We wanted to let you know that a [CICD pipeline](https://github.com/NVIDIA/NeMo/actions/runs/11587584218) for this PR just finished successfully

So it might be time to merge this PR or get some approvals

I'm just a bot so I'll leave it you what to do next.

//cc @pablo-garay @ko3n1g
# What does this PR do ?

support the case where monitor is the last metric in checkpoint name. for example 'global_step=1000'

# Usage
* You can potentially add a usage example below

```python
                run.Config(
                    ModelCheckpoint,
                    monitor='global_step',
                    filename='{global_step}',
                    every_n_train_steps=1000,
                    save_top_k=3,
                    mode='max',
                )
```


after fix: 
```
 0: [NeMo I 2024-10-28 01:06:24 model_checkpoint:310] Removed distributed checkpoint: nemo_experiments/default/checkpoints/global_step=70999.0
 0: [NeMo I 2024-10-28 01:06:24 model_checkpoint:310] Removed distributed checkpoint: nemo_experiments/default/checkpoints/global_step=71999.0
 0: [NeMo I 2024-10-28 01:06:24 model_checkpoint:310] Removed distributed checkpoint: nemo_experiments/default/checkpoints/global_step=72999.0
 0: [NeMo I 2024-10-28 01:06:24 model_checkpoint:310] Removed distributed checkpoint: nemo_experiments/default/checkpoints/global_step=73999.0
 0: [NeMo I 2024-10-28 01:06:24 model_checkpoint:310] Removed distributed checkpoint: nemo_experiments/default/checkpoints/global_step=74999.0
 ```
