Improve code readability and make number of epochs a command line argument

[lancerts]

Summary

• Make number of epochs a command line argument, the CI job runs 5 epoch (compared to its default value of 30) to reduce the overall testing time

• Improve code readability by separating the discriminator creation from the main function.
• const referece in const torch::data::Example<>& batch : *data_loader is standard (compared with the non-const version torch::data::Example<>& batch : *data_loader).
• fix warning C4244: 'initializing': conversion from 'double' to 'int64_t', possible loss of data and warning C4244: 'initializing': conversion from 'double' to 'const int64_t', possible loss of data
  with change
  const int64_t batches_per_epoch = static_cast<int64_t>(std::ceil(dataset.size().value() / static_cast<double>(kBatchSize)));

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[msaroufim]

a bit ugly no? Isn't there something like argparser in the cpp community?

[lancerts]

:P yeah, it is indeed awkward. Adopted the seemingly popular argparser and updated PR.