Consolidate BenchmarkMetric functionality in one file

Summary:
There is no need to have multiple files here anymore now that a lot of BenchmarkMetric functionality has disappeared.

D61736027 follows up by moving `benchmark/metrics/benchmark.py` to `benchmark/benchmark_metric.py` and moving the corresponding test file.

Reviewed By: Balandat

Differential Revision: D61432000

ax/benchmark/metrics/benchmark.py

@@ -5,13 +5,14 @@
 
 # pyre-strict
 
-from __future__ import annotations
-
 from typing import Any, Optional
 
-from ax.benchmark.metrics.utils import _fetch_trial_data
+import pandas as pd
 from ax.core.base_trial import BaseTrial
-from ax.core.metric import Metric, MetricFetchResult
+
+from ax.core.data import Data
+from ax.core.metric import Metric, MetricFetchE, MetricFetchResult
+from ax.utils.common.result import Err, Ok
 
 
 class BenchmarkMetric(Metric):
@@ -48,14 +49,59 @@ def __init__(
         self.outcome_index = outcome_index
 
     def fetch_trial_data(self, trial: BaseTrial, **kwargs: Any) -> MetricFetchResult:
+        """
+        Args:
+            trial: The trial from which to fetch data.
+            kwargs: Unsupported and will raise an exception.
+
+        Returns:
+            A MetricFetchResult containing the data for the requested metric.
+        """
         if len(kwargs) > 0:
             raise NotImplementedError(
                 f"Arguments {set(kwargs)} are not supported in "
                 f"{self.__class__.__name__}.fetch_trial_data."
             )
-        return _fetch_trial_data(
-            trial=trial,
-            metric_name=self.name,
-            outcome_index=self.outcome_index,
-            include_noise_sd=self.observe_noise_sd,
-        )
+        outcome_index = self.outcome_index
+        if outcome_index is None:
+            # Look up the index based on the outcome name under which we track the data
+            # as part of `run_metadata`.
+            outcome_names = trial.run_metadata.get("outcome_names")
+            if outcome_names is None:
+                raise RuntimeError(
+                    "Trials' `run_metadata` must contain `outcome_names` if "
+                    "no `outcome_index` is provided."
+                )
+            outcome_index = outcome_names.index(self.name)
+
+        try:
+            arm_names = list(trial.arms_by_name.keys())
+            all_Ys = trial.run_metadata["Ys"]
+            Ys = [all_Ys[arm_name][outcome_index] for arm_name in arm_names]
+
+            if self.observe_noise_sd:
+                stdvs = [
+                    trial.run_metadata["Ystds"][arm_name][outcome_index]
+                    for arm_name in arm_names
+                ]
+            else:
+                stdvs = [float("nan")] * len(Ys)
+
+            df = pd.DataFrame(
+                {
+                    "arm_name": arm_names,
+                    "metric_name": self.name,
+                    "mean": Ys,
+                    "sem": stdvs,
+                    "trial_index": trial.index,
+                }
+            )
+            return Ok(value=Data(df=df))
+
+        except Exception as e:
+            return Err(
+                MetricFetchE(
+                    message=f"Failed to obtain data for trial {trial.index}",
+                    exception=e,
+                )
+            )