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

pytorch3d/implicitron/dataset/sql_dataset.py

@@ -210,6 +210,9 @@ class SqlIndexDataset(DatasetBase, ReplaceableBase):  # pyre-ignore
             seq, frame = self._index.index[frame_idx]
         else:
             seq, frame, *rest = frame_idx
+            if isinstance(frame, torch.LongTensor):
+                frame = frame.item()
+
             if (seq, frame) not in self._index.index:
                 raise IndexError(
                     f"Sequence-frame index {frame_idx} not found; was it filtered out?"