handle no plugins found (#4969) (#4973)

Author: Chris Elion

ml-agents/mlagents/plugins/stats_writer.py

@@ -43,6 +43,14 @@ def register_stats_writer_plugins(run_options: RunOptions) -> List[StatsWriter]:
     and evaluates them, and returns the list of all the StatsWriter implementations.
     """
     all_stats_writers: List[StatsWriter] = []
+    if ML_AGENTS_STATS_WRITER not in importlib_metadata.entry_points():
+        logger.warning(
+            f"Unable to find any entry points for {ML_AGENTS_STATS_WRITER}, even the default ones. "
+            "Uninstalling and reinstalling ml-agents via pip should resolve. "
+            "Using default plugins for now."
+        )
+        return get_default_stats_writers(run_options)
+
     entry_points = importlib_metadata.entry_points()[ML_AGENTS_STATS_WRITER]
 
     for entry_point in entry_points: