Bug fix for step time

[NRauschmayr]

Description of changes:

I fixed the step time in PyTorch hook. It was only measuring the forward pass time.

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 #479 (23e80c8) into master (a7c697f) will decrease coverage by 0.64%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #479      +/-   ##
==========================================
- Coverage   66.88%   66.23%   -0.65%     
==========================================
  Files         173      163      -10     
  Lines       13280    12937     -343     
==========================================
- Hits         8882     8569     -313     
+ Misses       4398     4368      -30     

Impacted Files	Coverage Δ
smdebug/pytorch/hook.py	75.07% <100.00%> (-1.00%)	:arrow_down:
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/reduction_config.py	88.31% <0.00%> (-7.80%)	:arrow_down:
smdebug/core/reader.py	85.18% <0.00%> (-7.41%)	:arrow_down:
smdebug/tensorflow/callable_cache.py	82.60% <0.00%> (-6.53%)	:arrow_down:
smdebug/core/singleton_utils.py	85.29% <0.00%> (-5.89%)	:arrow_down:
... and 20 more

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