Fix Tensorboard Init (#763)
Fix Tensorboard Init (#763)

Summary:
Fix the mismatching between the parameter fed into `SummaryWriter` and the API of the latest [tensorboardX](https://github.com/lanpa/tensorboardX/blob/3e35c9b5f85e8ceb0294532d9eb772341a04c097/tensorboardX/writer.py#L192), i.e. "log_dir" -> "logdir".
Pull Request resolved: https://github.com/pytorch/fairseq/pull/763

Differential Revision: D15547192

Pulled By: myleott

fbshipit-source-id: c51b88da5ec589fb8ca5b4876bc229efeb7bf494

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq