Add stats_sender to MonaiAlgo for FL stats (#6984)

PR #6220 was closed and NVFlareStatsHandler has now been implemented in
NVFlare in NVIDIA/NVFlare#1925. However, there
is still the piece in MonaiAlgo to attach the stats_sender in
initialize, so this PR adds that missing piece.

### 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: Kevin <kevlu@nvidia.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

monai/fl/client/monai_algo.py

@@ -390,6 +390,7 @@ def __init__(
             if not isinstance(eval_workflow, BundleWorkflow) or eval_workflow.get_workflow_type() is None:
                 raise ValueError("train workflow must be BundleWorkflow and set type.")
             self.eval_workflow = eval_workflow
+        self.stats_sender = None
 
         self.app_root = ""
         self.filter_parser: ConfigParser | None = None
@@ -478,6 +479,12 @@ def initialize(self, extra=None):
         if len(config_filter_files) > 0:
             self.filter_parser.read_config(config_filter_files)
 
+        # set stats sender for nvflare
+        self.stats_sender = extra.get(ExtraItems.STATS_SENDER, self.stats_sender)
+        if self.stats_sender is not None:
+            self.stats_sender.attach(self.trainer)
+            self.stats_sender.attach(self.evaluator)
+
         # Get filters
         self.pre_filters = self.filter_parser.get_parsed_content(
             FiltersType.PRE_FILTERS, default=ConfigItem(None, FiltersType.PRE_FILTERS)

monai/fl/utils/constants.py

@@ -29,6 +29,7 @@ class ExtraItems(StrEnum):
     MODEL_TYPE = "fl_model_type"
     CLIENT_NAME = "fl_client_name"
     APP_ROOT = "fl_app_root"
+    STATS_SENDER = "fl_stats_sender"
 
 
 class FlPhase(StrEnum):