Fix unsupported data dtype for FindContours in vista2d

[KumoLiu]

Fixes #677

Description

Use cv2.convertScaleAbs function to scale and convert the label to an 8-bit format then apply find_contours, it will not influence the result of the contours.

Status

Ready/Work in progress/Hold

Please ensure all the checkboxes:

• Codeformat tests passed locally by running ./runtests.sh --codeformat.
• In-line docstrings updated.
• Update version and changelog in metadata.json if changing an existing bundle.
• Please ensure the naming rules in config files meet our requirements (please refer to: CONTRIBUTING.md).
• Ensure versions of packages such as monai, pytorch and numpy are correct in metadata.json.
• Descriptions should be consistent with the content, such as eval_metrics of the provided weights and TorchScript modules.
• Files larger than 25MB are excluded and replaced by providing download links in large_file.yml.
• Avoid using path that contains personal information within config files (such as use /home/your_name/ for "bundle_root").

[yiheng-wang-nv]

/build

[KumoLiu]

Hi @yiheng-wang-nv, let's wait for @SachidanandAlle and @myron's review to merge this PR, thanks!