[ONNX] Fix adding output nodes in NNCFGraph

[kshpv]

Changes

Do not add edges for the output node if it has no parents. E.g. output is put on the initializer.

Reason for changes

125248

Related tickets

125248

Tests

Checked on reproducer in the ticket

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 77.93%. Comparing base (0b407de) to head (fdc6897).

Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #2538       +/-   ##
============================================
- Coverage    91.16%   77.93%   -13.23%     
============================================
  Files          494      494               
  Lines        45373    45374        +1     
============================================
- Hits         41363    35363     -6000     
- Misses        4010    10011     +6001     

Files	Coverage Δ
nncf/onnx/graph/nncf_graph_builder.py	0.00% <0.00%> (-99.44%)	:arrow_down:

... and 104 files with indirect coverage changes

Flag	Coverage Δ
COMMON	?
ONNX	?
OPENVINO	?
TENSORFLOW	30.11% <0.00%> (-0.01%)	:arrow_down:
TORCH	65.94% <0.00%> (-0.01%)	:arrow_down:

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

Components	Coverage Δ
common	88.28% <ø> (-5.47%)	:arrow_down:
torch	93.48% <ø> (-0.01%)	:arrow_down:
tensorflow	93.74% <ø> (ø)
onnx	0.00% <0.00%> (-93.08%)	:arrow_down:
openvino	25.73% <ø> (-68.44%)	:arrow_down:
ptq	53.05% <ø> (-37.06%)	:arrow_down:

[kshpv]

LGTM. Please, add a test.

Added test

[kshpv]

E2E ONNX - 654 passed ONNX Conformance - 356 passed