[WIP] Add Flow1D Algorithm

[Zachary-66]

Motivation

Add Flow1D algorithm. This PR is based on #213 and the official code of Flow1D. Thanks for their excellent work!

Modification

1.configs/_base_/models/flow1d.py
2. configs/flow1d/flow1d_8xb2_100k_flyingchairs-368x496.py
3. mmflow/datasets/transforms/transforms.py (fix a bug)
4. mmflow/models/decoders/__init__.py
5. mmflow/models/decoders/flow1d_decoder.py
6. mmflow/models/flow_estimators/__init__.py
7. mmflow/models/flow_estimators/flow1d.py
8. mmflow/models/utils/__init__.py
9. mmflow/models/utils/attention1d.py
10. mmflow/models/utils/corr_lookup.py
11. mmflow/models/utils/correlation1d.py
12. tests/test_models/test_decoders/test_flow1d_decoder.py
13. tests/test_models/test_flow_estimators.py
14. tests/test_models/test_utils/test_corr_lookup.py

TODO

Reproduce the metrics of the original paper.

[codecov]

Codecov Report

❌ Patch coverage is 93.86792% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.19%. Comparing base (712ffdb) to head (1f5d98c).
⚠️ Report is 6 commits behind head on dev-1.x.

Files with missing lines	Patch %	Lines
mmflow/models/decoders/flow1d_decoder.py	90.00%	4 Missing and 6 partials ⚠️
mmflow/models/utils/attention1d.py	95.74%	0 Missing and 2 partials ⚠️
mmflow/datasets/transforms/transforms.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x     #255      +/-   ##
===========================================
+ Coverage    73.03%   75.19%   +2.16%     
===========================================
  Files           96      100       +4     
  Lines         4950     5160     +210     
  Branches       623      640      +17     
===========================================
+ Hits          3615     3880     +265     
+ Misses        1220     1155      -65     
- Partials       115      125      +10     

Flag	Coverage Δ
unittests	75.19% <93.86%> (+2.16%)	⬆️

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