fixed issue #2470 (some windows support for ffmpeg calls)

[dtraparic]

Motivation

Support on Windows

Modification

1. Added quotes around paths given to ffmpeg command (executed by mmcv.video.processing.py:convert_video() ), in order to support the spaces in windows paths.
2. Changed "which" to "where" when the platform is Windows in mmcv.utils.misc.py:_check_executable()

BC-breaking (Optional)

No

Use cases (Optional)

No new use case

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

None, I've tried to run the linting tools, but I got some exceptions that I didn't understand when trying to use them, maybe they don't work on Windows.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.

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