Fix bug for actions and improve design

[ndodda-amazon]

Description of changes:

Two major changes:

• Fixed a bug with actions where the message was not correctly passed (so SNS notifications would always send a message of None)
• Improved design so that all rules should either support actions or explicitly not support actions (by simply passing an action_str of ""). Going forward, if we create new rules, we won't forget to support actions.

Style and formatting:

I have run pre-commit install to ensure that auto-formatting happens with every commit.

Issue number, if available

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[codecov-io]

Codecov Report

Merging #448 (a79560d) into master (64fb95f) will increase coverage by 0.09%. The diff coverage is 76.92%.

@@            Coverage Diff             @@
##           master     #448      +/-   ##
==========================================
+ Coverage   65.73%   65.83%   +0.09%     
==========================================
  Files         161      161              
  Lines       12749    12747       -2     
==========================================
+ Hits         8381     8392      +11     
+ Misses       4368     4355      -13     

Impacted Files	Coverage Δ
smdebug/rules/action/stop_training_action.py	54.68% <50.00%> (+0.14%)	:arrow_up:
smdebug/rules/action/action.py	91.83% <100.00%> (ø)
smdebug/rules/action/message_action.py	81.92% <100.00%> (ø)
smdebug/rules/rule.py	84.84% <100.00%> (ø)
smdebug/core/reduction_config.py	88.31% <0.00%> (-2.60%)	:arrow_down:
smdebug/core/save_config.py	88.80% <0.00%> (+0.79%)	:arrow_up:
smdebug/core/writer.py	90.09% <0.00%> (+4.50%)	:arrow_up:
smdebug/core/logger.py	76.38% <0.00%> (+5.55%)	:arrow_up:
smdebug/core/tfevent/summary.py	88.13% <0.00%> (+6.77%)	:arrow_up:

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