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

projects/implicitron_trainer/impl/training_loop.py

@@ -249,6 +249,7 @@ class ImplicitronTrainingLoop(TrainingLoopBase):  # pyre-ignore [13]
         stats = Stats(
             # log_vars should be a list, but OmegaConf might load them as ListConfig
             list(log_vars),
+            plot_file=os.path.join(exp_dir, "train_stats.pdf"),
             visdom_env=visdom_env_charts,
             verbose=False,
             visdom_server=self.visdom_server,