fix optimizer pararmeter issue (#7322)

Fixes # .

### Description

A few sentences describing the changes proposed in this pull request.

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [ ] Documentation updated, tested `make html` command in the `docs/`
folder.

Signed-off-by: binliu <binliu@nvidia.com>

monai/handlers/mlflow_handler.py

@@ -401,7 +401,7 @@ def _default_iteration_log(self, engine: Engine) -> None:
             cur_optimizer = engine.optimizer
             for param_name in self.optimizer_param_names:
                 params = {
-                    f"{param_name} group_{i}": float(param_group[param_name])
+                    f"{param_name}_group_{i}": float(param_group[param_name])
                     for i, param_group in enumerate(cur_optimizer.param_groups)
                 }
                 self._log_metrics(params, step=engine.state.iteration)