code: def _mock_list_scalars(
      self,
      ctx,
      *,
      experiment_id,
      plugin_name,
      run_tag_filter=provider.RunTagFilter(),
  ):
      scalars_content = {
          "exp/session_1": {"loss": b"", "accuracy": b""},
          "exp/session_1/eval": {
              "loss": b"",
          },
          "exp/session_1/train": {
              "loss": b"",
          },
          "exp/session_2": {
              "loss": b"",
              "accuracy": b"",
          },
          "exp/session_2/eval": {
              "loss": b"",
          },
          "exp/session_2/train": {
              "loss": b"",
          },
          "exp/session_3": {
              "loss": b"",
              "accuracy": b"",
          },
          "exp/session_3/eval": {
              "loss": b"",
          },
          "exp/session_3xyz/": {
              "loss2": b"",
          },
      }
      result = {}
      for run, tag_to_content in scalars_content.items():
          result.setdefault(run, {})
          for tag, content in tag_to_content.items():
              t = provider.ScalarTimeSeries(
                  max_step=0,
                  max_wall_time=0,
                  plugin_content=content,
                  description="",
                  display_name="",
              )
              result[run][tag] = t
      return result
  
imports: import operator
  from unittest import mock
  import tensorflow
  from google.protobuf import text_format
  from tensorboard import context
  from tensorboard.data import provider
  from tensorboard.plugins import base_plugin
  from tensorboard.plugins.hparams import api_pb2
  from tensorboard.plugins.hparams import backend_context
  from tensorboard.plugins.hparams import metadata
  from tensorboard.plugins.hparams import plugin_data_pb2
label_desc: Use image node_id 3 for calling the BackendContextTest obj's underlying member method code with example usage: obj._mock_list_scalars(ctx) and returns: result