[IPU] Fix Custom Poptorch options to IPUPlugin

[SeanNaren]

What does this PR do?

When passing custom options to the IPU Plugin, I noticed that some of the options were incorrectly set leading to failures + options are not correctly set in poptorch.

This PR fixes this case where a custom IPUPlugin + options are passed in.

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[codecov]

Codecov Report

Merging #8241 (149e3a5) into master (3e6f884) will decrease coverage by 1%.
The diff coverage is 10%.

@@          Coverage Diff           @@
##           master   #8241   +/-   ##
======================================
- Coverage      93%     92%   -1%     
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  Files         212     212           
  Lines       13679   13707   +28     
======================================
- Hits        12720   12673   -47     
- Misses        959    1034   +75