Mods and bugfixes for LLFF and Blender repros

Summary:
LLFF (and most/all non-synth datasets) will have no background/foreground distinction. Add support for data with no fg mask.

Also, we had a bug in stats loading, like this:
  * Load stats
  * One of the stats has a history of length 0
  * That's fine, e.g. maybe it's fg_error but the dataset has no notion of fg/bg. So leave it as len 0
  * Check whether all the stats have the same history length as an arbitrarily chosen "reference-stat"
  * Ooops the reference-stat happened to be the stat with length 0
  * assert (legit_stat_len == reference_stat_len (=0)) ---> failed assert

Also some minor fixes (from Jeremy's other diff) to support LLFF

Reviewed By: davnov134

Differential Revision: D38475272

fbshipit-source-id: 5b35ac86d1d5239759f537621f41a3aa4eb3bd68

tests/implicitron/data/data_source.yaml

@@ -83,6 +83,7 @@ dataset_map_provider_LlffDatasetMapProvider_args:
   n_known_frames_for_test: null
   path_manager_factory_PathManagerFactory_args:
     silence_logs: true
+  downscale_factor: 4
 dataset_map_provider_RenderedMeshDatasetMapProvider_args:
   num_views: 40
   data_file: null

tests/implicitron/test_data_llff.py

@@ -69,6 +69,7 @@ class TestDataLlff(TestCaseMixin, unittest.TestCase):
         provider = LlffDatasetMapProvider(
             base_dir="manifold://co3d/tree/nerf_data/nerf_llff_data/fern",
             object_name="fern",
+            downscale_factor=8,
         )
         dataset_map = provider.get_dataset_map()
         known_matrix = torch.zeros(1, 4, 4)