Dump annotations with frame step: Key error

[mkostiv]

Steps to reproduce:

1. Create a task with frame step (5 in my case)
2. Annotate a few images
3. Dump annotations

git describe 0.2.0-750-g3f46258

Traceback (most recent call last):
File "/usr/local/lib/python3.5/dist-packages/rq/worker.py", line 812, in perform_job
rv = job.perform()
File "/usr/local/lib/python3.5/dist-packages/rq/job.py", line 588, in perform
self._result = self._execute()
File "/usr/local/lib/python3.5/dist-packages/rq/job.py", line 594, in _execute
return self.func(*self.args, **self.kwargs)
File "/home/django/cvat/apps/dataset_manager/views.py", line 87, in export_task_annotations
return export_task(task_id, dst_format, server_url=server_url, save_images=False)
File "/home/django/cvat/apps/dataset_manager/views.py", line 62, in export_task
server_url=server_url, save_images=save_images)
File "/home/django/cvat/apps/dataset_manager/task.py", line 723, in export_task
save_images=save_images)
File "/home/django/cvat/apps/dataset_manager/task.py", line 619, in export
exporter(dst_file, task_data, **options)
File "/home/django/cvat/apps/dataset_manager/formats/registry.py", line 35, in call
f_or_cls(*args, **kwargs)
File "/home/django/cvat/apps/dataset_manager/formats/cvat.py", line 545, in _export_images
anno_callback=dump_as_cvat_annotation, save_images=save_images)
File "/home/django/cvat/apps/dataset_manager/formats/cvat.py", line 517, in _export
anno_callback(f, task_data)
File "/home/django/cvat/apps/dataset_manager/formats/cvat.py", line 178, in dump_as_cvat_annotation
for frame_annotation in annotations.group_by_frame(include_empty=True):
File "/home/django/cvat/apps/dataset_manager/bindings.py", line 259, in group_by_frame
get_frame(shape['frame']).labeled_shapes.append(
File "/home/django/cvat/apps/dataset_manager/bindings.py", line 234, in get_frame
frame_info = self._frame_info[idx]
KeyError: 1

[nmanovic]

@zhiltsov-max , do you think you can look at the issue tomorrow?