[IPU] Allow poptorch.Options to override Trainer

[SeanNaren]

What does this PR do?

Follow-on from #8223. This will follow along from the conversation I had with @davors72, allowing the poptorch.Options that are passed into the IPUPlugin to be the source of truth, allowing the user to not have to pass any flags directly to the Trainer. I.e:

inference_opts = poptorch.Options()
inference_opts.replicationFactor(4)
inference_opts.deviceIterations(2)
inference_opts.Training.gradientAccumulation(1)

training_opts = poptorch.Options()
training_opts.replicationFactor(4)
training_opts.deviceIterations(2)
training_opts.Training.gradientAccumulation(2)

trainer = pl.Trainer(max_epochs=2, plugins=IPUPlugin(training_opts=training_opts, inference_opts=inference_opts))

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[codecov]

Codecov Report

Merging #8233 (0d6f8c5) into master (f3e74ab) will decrease coverage by 1%.
The diff coverage is 56%.

@@          Coverage Diff           @@
##           master   #8233   +/-   ##
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- Coverage      93%     92%   -1%     
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  Files         212     213    +1     
  Lines       13718   13803   +85     
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- Hits        12749   12737   -12     
- Misses        969    1066   +97