[Refactor] Change the meaning of momentum in EMA

[plyfager]

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Motivation

For aligning with MMEngine, we change the meaning of momentum in EMA. In past, we set momentum a number close to 1. and use it to multiply with ema_weights/ average_params.
Now we set momentum to a number close to 0., and use it to multiply with source_weights/ source_params.
This will cause the following changes:

1. The default value of momentum will be changed.
2. The momentum in configs will be changed.
3. The codes of EMAHook and AverageModel will be changed.

Notion:
Since RamupEMA's momentum won't be set in configs, so we only need to change the codes.

Modification

Please briefly describe what modification is made in this PR.

Who can help? @ them here!

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

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.
• CLA has been signed and all committers have signed the CLA in this PR.

[zengyh1900]

please also add the warning in the initialize function of average_model.py

[zengyh1900]

@plyfager please fix ut.

[codecov]

Codecov Report

Base: 87.92% // Head: 87.93% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (d7fe8c0) compared to base (328c875).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           dev-1.x    #1581   +/-   ##
========================================
  Coverage    87.92%   87.93%           
========================================
  Files          399      399           
  Lines        26051    26059    +8     
  Branches      4048     4051    +3     
========================================
+ Hits         22906    22914    +8     
  Misses        2240     2240           
  Partials       905      905           

Flag	Coverage Δ
unittests	87.93% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmedit/engine/hooks/ema.py	96.66% <100.00%> (+0.23%)	⬆️
mmedit/models/base_models/average_model.py	100.00% <100.00%> (ø)

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