[Fix] Fix docstring about noise in DINO

[nijkah]

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Motivation

rand_sign # [low, high), 1 or -1, randomly

mmdetection/mmdet/models/layers/transformer/dino_layers.py

Lines 410 to 413 in 78030a6

	# calculate the random part of the added noise
	rand_part = torch.rand_like(gt_bboxes_expand) # [0, 1)
	rand_part[negative_idx] += 1.0 # pos: [0, 1); neg: [1, 2)
	rand_part *= rand_sign # pos: (-1, 1); neg: (-2, 1] U [1, 2)

Current code is written as neg: (-2, 1] U [1, 2), but it is actually neg: (-2, -1] U [1, 2).

(Fig.3. of the original paper)

Modification

Fix neg: (-2, 1] U [1, 2) to neg: (-2, -1] U [1, 2).

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[jshilong]

LGTM

[Li-Qingyun]

@nijkah Good catch! Thanks for your pr!