fix argparse conflicting options error

[sidhantls]

What does this PR do?

Sets add_help=False in the parent_parser of semantic_segmentation in pl_examples. A conflicting options error occurs when running through command line because add_help=True in both the parent parser and the parser adding model specific args. Either one of them can be set to False to fix the error and still display the help options

Fixes #5568

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[codecov]

Codecov Report

Merging #5569 (bb62f67) into master (486f682) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #5569   +/-   ##
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- Coverage      93%     93%   -0%     
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  Files         135     135           
  Lines       10005   10005           
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- Hits         9339    9332    -7     
- Misses        666     673    +7