Fixed Siammask tracker error on grayscale images (#6982)

```python
"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)
```

changelog.d/20231011_153251_andrey_fix_siammask.md

@@ -0,0 +1,4 @@
+### Fixed
+
+- Fixed Siammask tracker error on grayscale images
+  (<https://github.com/opencv/cvat/pull/6982>)

serverless/pytorch/foolwood/siammask/nuclio/main.py

@@ -20,6 +20,7 @@ def handler(context, event):
     shapes = data.get("shapes")
     states = data.get("states")
     image = Image.open(buf)
+    image = image.convert("RGB")  #  to make sure image comes in RGB
 
     results = {
         'shapes': [],