[Fix] Refine ActionDataSample structure

[Dai-Wenxun]

For BC-breaking:

• replace data_sample.pred_scores.item with data_sample.pred_score
• replace data_sample.gt_labels.item with data_sample.gt_label
• replace data_sample.set_gt_labels() with data_sample.set_gt_label()

[codecov]

Codecov Report

Patch coverage is 46.84% of modified lines.

❗ Current head a7a5dd9 differs from pull request most recent head 326a8d4. Consider uploading reports for the commit 326a8d4 to get more accurate results

Files Changed	Coverage
mmaction/apis/__init__.py	ø
...ion/models/data_preprocessors/data_preprocessor.py	0.00%
mmaction/models/heads/rgbpose_head.py	0.00%
mmaction/models/recognizers/base.py	ø
mmaction/apis/inference.py	14.70%
mmaction/evaluation/metrics/acc_metric.py	44.82%
mmaction/models/losses/hvu_loss.py	50.00%
mmaction/structures/action_data_sample.py	76.47%
...maction/apis/inferencers/actionrecog_inferencer.py	100.00%
mmaction/datasets/transforms/formatting.py	100.00%
... and 7 more

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