`CellTracker.track_cells` crashes if there is a frame with no objects.

[willgraf]

I've encountered an issue where track_cells crashes unexpectedly with this error:

Traceback (most recent call last):
  File "/train.py", line 324, in <module>
    track_length=args.track_length)
  File "/train.py", line 242, in evaluate
    tracker.track_cells()
  File "/usr/local/lib/python3.6/dist-packages/deepcell_tracking/tracking.py", line 639, in track_cells
    self._track_frame(frame)
  File "/usr/local/lib/python3.6/dist-packages/deepcell_tracking/tracking.py", line 623, in _track_frame
    cost_matrix, predictions = self._get_cost_matrix(frame)
  File "/usr/local/lib/python3.6/dist-packages/deepcell_tracking/tracking.py", line 420, in _get_cost_matrix
    ], axis=0)
  File "<__array_function__ internals>", line 6, in stack
  File "/usr/local/lib/python3.6/dist-packages/numpy/core/shape_base.py", line 423, in stack
    raise ValueError('need at least one array to stack')
ValueError: need at least one array to stack

Digging into the error, I find that in _get_cost_matrix, we compare the current cell features with the future cell features, stacking together each feature into a single array. The error indicates that we are trying to stack an empty list, which means that future_feature is empty.

deepcell-tracking/deepcell_tracking/tracking.py

Lines 414 to 420 in 526b65b

	# Convert from dict to arrays
	current_feature_arr = np.stack([
	current_feature[k] for k in current_feature
	], axis=1) # time axis already included
	future_feature_arr = np.stack([
	future_feature[k] for k in future_feature
	], axis=0)

future_feature is initialized by _get_frame_features, which populates a dictionary with features for every cell in the frame. I think the root cause of this bug is that there are no objects in the frame, and so future_feature is returned as an empty dictionary.

deepcell-tracking/deepcell_tracking/tracking.py

Lines 269 to 273 in 526b65b

	frame_features = {}
	for cell_id in cells_in_frame:
	f = self._get_feature(frame, cell_id, feature_name=feature_name)
	frame_features[cell_id] = f
	return frame_features

It would also be prudent make sure that _fetch_tracked_features (which creates the current_feature dictionary that also gets stacked together) is also robust to empty frames.

[willgraf]

The Track object should also be investigated for this empty frame issue.