[Tests] Remove raising error when graph is not existed

[kshpv]

Changes

Remove raising an error in the test when a graph does not exist.

Reason for changes

To return the feature with re/generating reference graphs using NNCF_TEST_REGEN_DOT

Related tickets

N/A

Tests

N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.19%. Comparing base (85b3263) to head (7048307).
Report is 5 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2744      +/-   ##
===========================================
+ Coverage    91.18%   91.19%   +0.01%     
===========================================
  Files          483      483              
  Lines        46363    46443      +80     
===========================================
+ Hits         42274    42355      +81     
+ Misses        4089     4088       -1     

see 13 files with indirect coverage changes

Flag	Coverage Δ
COMMON	41.93% <ø> (-0.10%)	⬇️
ONNX	34.18% <ø> (-0.01%)	⬇️
OPENVINO	40.96% <ø> (+0.11%)	⬆️
TENSORFLOW	29.39% <ø> (-0.04%)	⬇️
TORCH	65.37% <ø> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
common	93.55% <ø> (+<0.01%)	⬆️
torch	93.65% <ø> (ø)
tensorflow	93.26% <ø> (ø)
onnx	93.06% <ø> (ø)
openvino	94.48% <ø> (+<0.01%)	⬆️
ptq	90.50% <ø> (+0.06%)	⬆️