Change hook_cfg type access to hook_cfg.get()

[lorinczszabolcs]

Motivation

Python dictionary does not support dot notation to access values. Fixes #396

Modification

Changed hook_cfg.type to hook_cfg.get('type', None).

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 #409 (26e6702) into dev (171a475) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##              dev     #409   +/-   ##
=======================================
  Coverage   72.47%   72.47%           
=======================================
  Files         121      121           
  Lines        4759     4759           
  Branches      763      763           
=======================================
  Hits         3449     3449           
  Misses       1151     1151           
  Partials      159      159           

Flag	Coverage Δ
unittests	72.47% <0.00%> (ø)

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

Impacted Files	Coverage Δ
mmselfsup/apis/train.py	47.77% <0.00%> (ø)

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[fangyixiao18]

LGTM.