Fix type casting in compute_segments
Author: achsvg
Pull request: yes

# What does this PR do?
Fix bug in `feature_extraction_detr.py` compute_segments function: make sure the shape is integer (it could be float)

If the shape is float, there will be an error message:

```
zeros() received an invalid combination of arguments - got (tuple, device=torch.device, dtype=torch.dtype), but expected one of:
* (tuple of ints size, *, tuple of names names, torch.dtype dtype, torch.layout layout, torch.device device, bool pin_memory, bool requires_grad)
* (tuple of ints size, *, Tensor out, torch.dtype dtype, torch.layout layout, torch.device device, bool pin_memory, bool requires_grad)
```
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Fixes # (issue)


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