[Refactor]Resume do not specify ckpt

[MeowZheng]

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Motivation

resume is only used to specify whether launch resume training from the latest checkpoint and load-from is used to load specific checkpoints.

resume in CLI has the same function of resume in Runner. https://github.com/open-mmlab/mmengine/blob/ca0364bbc3a6ac9132f5d499de190f7e733383ca/mmengine/runner/runner.py#L151-L154

Modification

1. Add action of resume.
2. Remove the load ckpt from resume.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
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

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMDet3D.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Base: 83.10% // Head: 83.10% // No change to project coverage 👍

Coverage data is based on head (e8626e5) compared to base (fffb9e2).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff            @@
##           dev-1.x    #2074   +/-   ##
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  Coverage    83.10%   83.10%           
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  Files          138      138           
  Lines         7779     7779           
  Branches      1246     1246           
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  Hits          6465     6465           
  Misses        1130     1130           
  Partials       184      184           

Flag	Coverage Δ
unittests	83.10% <ø> (ø)

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