Save Nested Layers For Rubik

[NihalHarish]

Description of changes:

• This PR is in the draft stage, I need to update tests, refactor and add comments.
• Adds ability to save nested layers with the use of the _flatten_layers API. See tests/tensorflow2/test_nested_layers.py
• Adds the ability to save repeated calls to the same layer object with model subclassing by storing the outputs of the layers in a list in the InputOutputSaver object. See tests/tensorflow2/test_model_that_reuses_layers.py
• Creates a unique name for each layer output and input to make it more intuitive for the user. See tests/tensorflow2/test_concat_layer.py

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 #377 into master will decrease coverage by 2.72%. The diff coverage is 98.03%.

@@            Coverage Diff             @@
##           master     #377      +/-   ##
==========================================
- Coverage   85.49%   82.76%   -2.73%     
==========================================
  Files          86       86              
  Lines        6514     6539      +25     
==========================================
- Hits         5569     5412     -157     
- Misses        945     1127     +182     

Impacted Files	Coverage Δ
smdebug/tensorflow/keras.py	80.03% <97.77%> (-12.29%)	:arrow_down:
smdebug/tensorflow/utils.py	63.45% <100.00%> (-24.88%)	:arrow_down:
smdebug/tensorflow/singleton_utils.py	83.33% <0.00%> (-16.67%)	:arrow_down:
smdebug/tensorflow/callable_cache.py	69.56% <0.00%> (-13.05%)	:arrow_down:
smdebug/tensorflow/collection.py	84.53% <0.00%> (-11.35%)	:arrow_down:
smdebug/tensorflow/tensor_ref.py	82.25% <0.00%> (-6.46%)	:arrow_down:
smdebug/tensorflow/base_hook.py	76.19% <0.00%> (-4.37%)	:arrow_down:
smdebug/tensorflow/session.py	88.46% <0.00%> (-3.37%)	:arrow_down:
... and 9 more

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