fix BUG of SSDHead when predict

[zzj403]

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Motivation

I meet a bug when predict with ssd
'SSDHead' object has no attribute 'loss_cls'
on mmdet/models/dense_heads/base_dense_head.py L376:

if getattr(self.loss_cls, 'custom_cls_channels', False):

Modification

I fix it by change this line to:

if hasattr(self, 'loss_cls') and getattr(self.loss_cls, 'custom_cls_channels', False):

Use cases (Optional)

from mmdet.apis import DetInferencer

inferencer = DetInferencer('ssd512_coco')

inferencer('demo/demo.jpg', show=False, out_dir='demo/result')

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 MMPreTrain.
4. The documentation has been modified accordingly, like docstring or example tutorials.

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