[Features] Load optical flow data from bytes

[MeowZheng]

Motivation

To support optical flow data loading from different storage platforms, I implement two functions to decode optical flow from bytes.

Modification

1. Add flow_from_bytes and sparse_flow_from_bytes functions in mmcv/video/optflow.py
2. Revise mmcv/video/__init__.py
3. Add unit test in tests/test_video/test_optflow.py

BC-breaking (Optional)

no

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.

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.

[zhouzaida]

LGTM

[ZwwWayne]

Last comment: the tests/data/sparse_flow.png is a little big, better to compress it to a size smaller than 100KB.

[codecov]

Codecov Report

Merging #1362 (2525b5f) into master (4bab292) will decrease coverage by 0.06%.
The diff coverage is 88.23%.

@@            Coverage Diff             @@
##           master    #1362      +/-   ##
==========================================
- Coverage   69.14%   69.07%   -0.07%     
==========================================
  Files         162      162              
  Lines       10746    10776      +30     
  Branches     1978     1983       +5     
==========================================
+ Hits         7430     7444      +14     
- Misses       2927     2940      +13     
- Partials      389      392       +3     

Flag	Coverage Δ
unittests	69.07% <88.23%> (-0.07%)	⬇️

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Impacted Files	Coverage Δ
mmcv/video/optflow.py	96.07% <87.50%> (-1.60%)	⬇️
mmcv/video/__init__.py	100.00% <100.00%> (ø)
mmcv/ops/pixel_group.py	72.72% <0.00%> (-27.28%)	⬇️
mmcv/ops/contour_expand.py	75.00% <0.00%> (-25.00%)	⬇️
mmcv/utils/ext_loader.py	35.89% <0.00%> (ø)

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