[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 #8223 (ff0c8d7) into master (8b0aec8) will decrease coverage by 1%.
The diff coverage is 10%.

@@          Coverage Diff           @@
##           master   #8223   +/-   ##
======================================
- Coverage      93%     92%   -1%     
======================================
  Files         212     212           
  Lines       13703   13712    +9     
======================================
- Hits        12743   12678   -65     
- Misses        960    1034   +74     

[SeanNaren]

This has been moved to #8241 as the caching in the CI was breaking the test.