Add Support For More TF Types

[NihalHarish]

Description of changes:

• Add support for any TF Type that has a TF to Numpy mapping

• Add a test to check if the mapping dict becomes stale

• A bug that was recently reported was because the assumption that tf tensors are mapped to a single numpy type is wrong. For example, tf string can have npdtype.kind == "U" or npdtype.kind == "O" or npdtype.kind == "S" ( unicode, object, str).

• The user does not control this low level detail and is determined by factors like training mode, dataset et cetera.

• This PR introduces a test that is run with the _NP_TO_TF datastructure from tensorflow.python.framework.dtypes

• It will break if a new numpy datatype support is added, prompting the smdebug project to also sync

  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 #330 into master will decrease coverage by 8.06%. The diff coverage is 92.63%.

@@            Coverage Diff             @@
##           master     #330      +/-   ##
==========================================
- Coverage   84.96%   76.90%   -8.07%     
==========================================
  Files          86       86              
  Lines        6367     6409      +42     
==========================================
- Hits         5410     4929     -481     
- Misses        957     1480     +523     

Impacted Files	Coverage Δ
smdebug/core/hook.py	85.86% <73.33%> (-7.38%)	:arrow_down:
smdebug/tensorflow/keras.py	79.60% <94.23%> (-13.99%)	:arrow_down:
smdebug/_version.py	100.00% <100.00%> (ø)
smdebug/core/config_constants.py	100.00% <100.00%> (ø)
smdebug/core/tfevent/util.py	92.50% <100.00%> (+8.62%)	:arrow_up:
smdebug/tensorflow/base_hook.py	74.62% <100.00%> (-4.00%)	:arrow_down:
smdebug/tensorflow/constants.py	100.00% <100.00%> (ø)
smdebug/tensorflow/utils.py	62.63% <100.00%> (-25.14%)	:arrow_down:
smdebug/mxnet/__init__.py	0.00% <0.00%> (-100.00%)	:arrow_down:
smdebug/mxnet/singleton_utils.py	0.00% <0.00%> (-100.00%)	:arrow_down:
... and 34 more

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