fix: use torch.as_tensor for Mask-RCNN labels to support multiple classes

[shoushinya123]

Fixes #960

Change labels = torch.ones((num_objs,), dtype=torch.int64) to labels = torch.as_tensor(obj_ids, dtype=torch.int64) in the Mask-RCNN tutorial.

This fix correctly maps object IDs to labels, supporting datasets with more than two object instance types instead of assigning all labels to 1.

[pytorch-bot]

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