Fix for BEVFusion HungarianAssigner3D

[ScriptumVeritas]

Motivation

In case of no ground truth, the HungarianAssigner3D in the BEVFusion project returns 'None' instead of a tensor.
This will rise an error in the following code of the transfusion_head.py running:
max_overlaps=torch.cat([res.max_overlaps for res in assign_result_list]),

Modification

return a tensor with zeros for max_overlaps if there is no ground truth or no bounding boxes. In case both are 0, an empty tensor is returned which still provides better compatibility

Use cases (Optional)

no error in cases where no ground truth or no bounding box is given.

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1 out of 2 committers have signed the CLA.

✅ chbehrens
❌ Fabian

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