fix bug when using iterbaserunner with 'val' workflow

[Ezra-Yu]

Motivation

fix bug when using iterbaserunner with 'val' workflow. refer to #535
Align the train_step and val_step with epochBaseRunner and iterBaseRunner in mmcv

Modification

add a default value None for optimizer
add kwargs

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

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

[codecov]

Codecov Report

Merging #542 (a498f07) into master (771d105) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #542   +/-   ##
=======================================
  Coverage   79.87%   79.88%           
=======================================
  Files         107      107           
  Lines        6093     6095    +2     
  Branches      987      988    +1     
=======================================
+ Hits         4867     4869    +2     
  Misses       1094     1094           
  Partials      132      132           

Flag	Coverage Δ
unittests	79.88% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
mmcls/models/classifiers/base.py	69.62% <100.00%> (ø)
mmcls/datasets/pipelines/transforms.py	88.22% <0.00%> (+0.04%)	⬆️

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[mzr1996]

Well, the right usage should be

Suggested change

	optimizer (:obj:`torch.optim.Optimizer` | dict | optional): The
	optimizer (:obj:`torch.optim.Optimizer` | dict, optional): The

Because optional is not a type, but a kind of annotation.

[mzr1996]

LGTM