[DO Not Merge] Avoid using 'default' keyword while invoking 'get' method

[leleamol]

Description of changes:

As mentioned in the issue #418 customers may see the error because of having 'default' keyword in the 'get()' call.

This change avoid using this keyword argument in 'get()' call to avoid potential issues.

As mentioned in the issue, the tokenizer in transformers package is modifying the data type of os.environ from os.Environ to 'dict'. The change in this PR is for protecting smdebug from such changes while keeping the behavior same.

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-io]

Codecov Report

Merging #473 (f9b09ae) into master (1aac94b) will decrease coverage by 0.44%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #473      +/-   ##
==========================================
- Coverage   65.53%   65.09%   -0.45%     
==========================================
  Files         173      163      -10     
  Lines       13280    12934     -346     
==========================================
- Hits         8703     8419     -284     
+ Misses       4577     4515      -62     

Impacted Files	Coverage Δ
smdebug/core/logger.py	70.83% <100.00%> (ø)
smdebug/core/modes.py	55.00% <0.00%> (-20.00%)	:arrow_down:
smdebug/xgboost/singleton_utils.py	0.00% <0.00%> (-20.00%)	:arrow_down:
smdebug/trials/profiler_trial.py	24.61% <0.00%> (-18.47%)	:arrow_down:
smdebug/mxnet/collection.py	73.33% <0.00%> (-16.67%)	:arrow_down:
smdebug/exceptions.py	65.47% <0.00%> (-15.48%)	:arrow_down:
smdebug/core/reader.py	85.18% <0.00%> (-7.41%)	:arrow_down:
smdebug/tensorflow/callable_cache.py	78.26% <0.00%> (-6.53%)	:arrow_down:
smdebug/core/locations.py	86.11% <0.00%> (-5.56%)	:arrow_down:
smdebug/core/tensor.py	79.03% <0.00%> (-2.02%)	:arrow_down:
... and 12 more

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[NihalHarish]

Let's not merge this. If this is the correct way to handle env variables then be it.