[IPU] Remove convert_to_half, suggest using model.half

[SeanNaren]

What does this PR do?

As discussed in #7923 use model.half for pure FP16 training rather than having a specific argument for the plugin.

Fixes #<issue_number>

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[codecov]

Codecov Report

Merging #7974 (47d4a4d) into master (f7459f5) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #7974   +/-   ##
======================================
- Coverage      92%     91%   -0%     
======================================
  Files         203     203           
  Lines       13157   13151    -6     
======================================
- Hits        12043   12004   -39     
- Misses       1114    1147   +33