Test Concat Layers

[NihalHarish]

Description of changes:

• Corrects and incorrect type check that caused bugs when trying to save layer that have multiple inputs and outputs
• Test to prevent this bug.

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-commenter]

Codecov Report

Merging #367 into master will increase coverage by 4.45%. The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #367      +/-   ##
==========================================
+ Coverage   81.03%   85.48%   +4.45%     
==========================================
  Files          86       86              
  Lines        6501     6504       +3     
==========================================
+ Hits         5268     5560     +292     
+ Misses       1233      944     -289     

Impacted Files	Coverage Δ
smdebug/tensorflow/keras.py	92.32% <85.71%> (-0.14%)	:arrow_down:
smdebug/trials/trial.py	96.82% <0.00%> (+0.28%)	:arrow_up:
smdebug/core/utils.py	84.03% <0.00%> (+0.46%)	:arrow_up:
smdebug/exceptions.py	67.53% <0.00%> (+1.29%)	:arrow_up:
smdebug/core/reductions.py	93.47% <0.00%> (+2.17%)	:arrow_up:
smdebug/core/save_config.py	88.00% <0.00%> (+2.40%)	:arrow_up:
smdebug/core/hook.py	93.45% <0.00%> (+5.35%)	:arrow_up:
smdebug/core/state_store.py	98.87% <0.00%> (+5.61%)	:arrow_up:
smdebug/rules/action/stop_training_action.py	77.41% <0.00%> (+24.19%)	:arrow_up:
smdebug/mxnet/graph.py	25.80% <0.00%> (+25.80%)	:arrow_up:
... and 7 more

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