Fuse boxes decoding for `LogitsDecoder`

[zhiqwang]

🚀 The feature

Seems that the boxes decoding computation can be fused at:
https://github.com/zhiqwang/yolov5-rt-stack/blob/fb7e12039a9af3a37d1d6ea61505ca39aecf349f/yolort/relay/logits_decoder.py#L49-L60

Motivation, pitch

Accelerated computation for LogitsDecoder.

Alternatives

No response

Additional context

No response