Logging model checkpoints as artifacts in MlFlow 
# What does this PR do ?

MlFlow is required for training and resuming runs from checkpoint. Currently, when the MlFlow logger is enabled in experiment manager it doesn't log model checkpoint as artifacts to MLFlow.
This PR simply adds one line to the `nemo.utils.loggers.MLFlowParams` to allow for logging checkpoints.

**Collection**: [Note which collection this PR will affect]

# Changelog 
- Add `log_model ` to `nemo.utils.loggers.MLFlowParams`

# Usage
To enable logging checkpoints in MlFlow
```python
exp_manager:
  ...
  create_checkpoint_callback: True
  create_mlflow_logger: True
  mlflow_logger_kwargs:
  ...
      log_model: True
```


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**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation

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# Additional Information
* Related to #7257 
