Model Subclassing Test

[NihalHarish]

Description of changes:

• Three types of Model exist: subclassed Model, Functional model, and Sequential (a special case of Functional).
• This PR adds a test a for subclassed Model
• The code that saved intermediate layer values is present in the run_internal_graph function for Functional models in AWS TF.
• This PR adds support to save intermediate layer values for subclassed models.

Style and formatting:

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

Issue number, if available

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

Merging #351 into master will increase coverage by 0.35%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #351      +/-   ##
==========================================
+ Coverage   85.18%   85.53%   +0.35%     
==========================================
  Files          86       86              
  Lines        6466     6466              
==========================================
+ Hits         5508     5531      +23     
+ Misses        958      935      -23     

Impacted Files	Coverage Δ
smdebug/tensorflow/keras.py	93.07% <100.00%> (+0.17%)	:arrow_up:
smdebug/pytorch/utils.py	18.51% <0.00%> (-3.71%)	:arrow_down:
smdebug/mxnet/hook.py	93.02% <0.00%> (-2.33%)	:arrow_down:
smdebug/core/reduction_config.py	88.31% <0.00%> (-1.30%)	:arrow_down:
smdebug/trials/trial.py	96.82% <0.00%> (-0.58%)	:arrow_down:
smdebug/core/utils.py	83.56% <0.00%> (-0.47%)	:arrow_down:
smdebug/pytorch/hook.py	82.40% <0.00%> (ø)
smdebug/tensorflow/base_hook.py	80.55% <0.00%> (+1.58%)	:arrow_up:
smdebug/mxnet/collection.py	76.66% <0.00%> (+3.33%)	:arrow_up:
smdebug/rules/action/message_action.py	85.54% <0.00%> (+3.61%)	:arrow_up:
... and 4 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update b515fe3...d091d5c. Read the comment docs.