Refactor: FrameDataBuilder is more extensible.

Summary:
This is mostly a refactoring diff to reduce friction in extending the frame data.

Slight functional changes: dataset getitem now accepts (seq_name, frame_number_as_singleton_tensor) as a non-advertised feature. Otherwise this code crashes:
```
item = dataset[0]
dataset[item.sequence_name, item.frame_number]
```

Reviewed By: bottler

Differential Revision: D45780175

fbshipit-source-id: 75b8e8d3dabed954a804310abdbd8ab44a8dea29

projects/implicitron_trainer/tests/test_experiment.py

@@ -132,6 +132,11 @@ class TestExperiment(unittest.TestCase):
         # Check that the default config values, defined by Experiment and its
         # members, is what we expect it to be.
         cfg = OmegaConf.structured(experiment.Experiment)
+        # the following removes the possible effect of env variables
+        ds_arg = cfg.data_source_ImplicitronDataSource_args
+        ds_arg.dataset_map_provider_JsonIndexDatasetMapProvider_args.dataset_root = ""
+        ds_arg.dataset_map_provider_JsonIndexDatasetMapProviderV2_args.dataset_root = ""
+        cfg.training_loop_ImplicitronTrainingLoop_args.visdom_port = 8097
         yaml = OmegaConf.to_yaml(cfg, sort_keys=False)
         if DEBUG:
             (DATA_DIR / "experiment.yaml").write_text(yaml)