Compatibility with TF 2.7

[adimux]

...by fixing broken tests in test_keras_mirrored.py

Motive: Tensorflow 2.7.0 release

smdebug.tensorflow.get_hook() was returning None, which made many tests fail, but this was actually a false alarm. We just have to mark the version of tensorflow we are trying to release (2.7.0) as a supported framework version, so get_hook() doesn't return `None'...

I know what you think. It's not pythonic to silently fail and we should instead throw an exception, but this code preceded me and I don't want to introduce a breaking change (an exception being raised) and I don't want to introduce warnings either in case `get_hook()' gets called in unexpected ways multiple times during the lifecycle of a training job.

Instead, I just created a tests that alarms us of the situation when get_hook() might return None when it's actually just because we forgot to update a constant.

Description of changes:

Style and formatting:

I have run pre-commit install && pre-commit run --all-files 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 #526 (3927d20) into master (88be531) will decrease coverage by 43.71%. The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           master     #526       +/-   ##
===========================================
- Coverage   74.70%   30.98%   -43.72%     
===========================================
  Files         127      117       -10     
  Lines       11111    10591      -520     
===========================================
- Hits         8300     3282     -5018     
- Misses       2811     7309     +4498     

Impacted Files	Coverage Δ
smdebug/core/utils.py	60.98% <0.00%> (-19.84%)	:arrow_down:
smdebug/pytorch/utils.py	0.00% <0.00%> (-57.15%)	:arrow_down:
smdebug/tensorflow/singleton_utils.py	0.00% <ø> (-94.12%)	:arrow_down:
smdebug/tensorflow/utils.py	0.00% <0.00%> (-86.18%)	:arrow_down:
smdebug/pytorch/__init__.py	0.00% <0.00%> (-100.00%)	:arrow_down:
smdebug/tensorflow/__init__.py	0.00% <0.00%> (-100.00%)	:arrow_down:
smdebug/tensorflow/constants.py	0.00% <0.00%> (-100.00%)	:arrow_down:
smdebug/core/config_validator.py	0.00% <0.00%> (-100.00%)	:arrow_down:
smdebug/pytorch/singleton_utils.py	0.00% <0.00%> (-100.00%)	:arrow_down:
...ug/profiler/analysis/utils/pandas_data_analysis.py	0.00% <0.00%> (-98.43%)	:arrow_down:
... and 83 more

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[adimux]

Our pipeline runs with an outdated version of Tensorflow (2.4). I will fix the pipeline later, but right now we need to unblock the release of 2.7. Fortunately, the DLC pipelines do run the sagemaker-debugger tests and I also ran them locally and they're green.

[hongshanli23]

LGTM

[abhinavs95]

LGTM

[adimux]

created other PR to be able to merge