Fix TPU Convergence bug introduced by PR#6151 (#6488)
Fix TPU Convergence bug introduced by PR#6151 (#6488)

Currently with the bug introduced we're taking two optimizer steps per
batch: one global one, where `xm.optimizer_step` injects a CRS between
all cores in training, and one without. This has been affecting training
accuracy (for example, XLNet GLUE on MNLI is not converging, etc.).
huggingface/transformers,huggingface/transformers,huggingface/transformers,huggingface/transformers,huggingface/pytorch-transformers