Fixed Siammask tracker error on grayscale images

[azhavoro]

Motivation and context

"Exception caught in handler - \"not enough values to unpack (expected 3, got 2)\": Traceback (most recent call last):
 File \"/opt/nuclio/_nuclio_wrapper.py\", line 143, in serve_requests
 await self._handle_event(event)
 File \"/opt/nuclio/_nuclio_wrapper.py\", line 397, in _handle_event
 entrypoint_output = self._entrypoint(self._context, event)
 File \"/opt/nuclio/main.py\", line 29, in handler
 shape, state = context.user_data.model.infer(image, shape, states[i] if i < len(states) else None)
 File \"/opt/nuclio/model_handler.py\", line 56, in infer
 self.config['hp'], device=self.device)
 File \"/opt/nuclio/SiamMask/tools/test.py\", line 152, in siamese_init
 z_crop = get_subwindow_tracking(im, target_pos, p.exemplar_size, s_z, avg_chans)
 File \"/opt/nuclio/SiamMask/tools/test.py\", line 88, in get_subwindow_tracking
 r, c, k = im.shape
ValueError: not enough values to unpack (expected 3, got 2)

How has this been tested?

Manually

Checklist

• I submit my changes into the develop branch
• I have created a changelog fragment
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (see GitHub docs)
• I have increased versions of npm packages if it is necessary
  (cvat-canvas,
  cvat-core,
  cvat-data and
  cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.

[codecov]

Codecov Report

Merging #6982 (1381026) into develop (e699848) will decrease coverage by 0.01%.
Report is 13 commits behind head on develop.
The diff coverage is 90.00%.

@@             Coverage Diff             @@
##           develop    #6982      +/-   ##
===========================================
- Coverage    82.58%   82.57%   -0.01%     
===========================================
  Files          360      360              
  Lines        38914    38927      +13     
  Branches      3570     3570              
===========================================
+ Hits         32137    32144       +7     
- Misses        6777     6783       +6     

Components	Coverage Δ
cvat-ui	77.59% <90.00%> (-0.01%)	⬇️
cvat-server	87.14% <ø> (-0.01%)	⬇️