[tune/dashboard] Fix Tune dashboard to work with all trainables (#11232)

python/ray/dashboard/dashboard.py

@@ -811,7 +811,7 @@ class TuneCollector(threading.Thread):
 
         # search through all the sub_directories in log directory
         analysis = Analysis(str(self._logdir))
-        df = analysis.dataframe(metric="episode_reward_mean", mode="max")
+        df = analysis.dataframe(metric=None, mode=None)
 
         if len(df) == 0 or "trial_id" not in df.columns:
             return

python/ray/tune/analysis/experiment_analysis.py

@@ -91,8 +91,11 @@ class Analysis:
         Returns:
             pd.DataFrame: Constructed from a result dict of each trial.
         """
-        metric = self._validate_metric(metric)
-        mode = self._validate_mode(mode)
+        # Allow None values here.
+        if metric or self.default_metric:
+            metric = self._validate_metric(metric)
+        if mode or self.default_mode:
+            mode = self._validate_mode(mode)
 
         rows = self._retrieve_rows(metric=metric, mode=mode)
         all_configs = self.get_all_configs(prefix=True)