UPDATE ConfusionMatrix support multimodal predicts

[HoBeom]

Motivation

#2815

Modification

acc_metric.py/class ConfusionMatrix

BC-breaking (Optional)

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Use cases (Optional)

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Checklist

1. Pre-commit or other linting tools should be used to fix the potential lint issues. ✅
2. The modification should be covered by complete unit tests. If not, please add more unit tests 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 should be modified accordingly, like docstring or example tutorials. ⛔︎

[HoBeom]

Also I got error in RGBPoseHead in training on last batch with less then batch size.

mmaction2/mmaction/models/heads/rgbpose_head.py

Lines 117 to 118 in 4d6c934

	elif labels.dim() == 1 and labels.size()[0] == self.num_classes \
	and cls_scores.size()[0] == 1:

can be changed as follows to check the size correctly.

        elif labels.dim() == 1 and labels.size()[0] == self.num_classes \
                and cls_scores[self.loss_components[0]].size()[0] == 1: