Add more dataloader options to the trainer for TPU training. 
🚀 Feature
Add more dataloader options to the trainer for TPU training.
Motivation
As is described in https://pytorch.org/xla/release/1.5/_modules/torch_xla/distributed/parallel_loader.html, the signature for ParallelLoader is
  def __init__(self,
               loader,
               devices,
               batchdim=0,
               fixed_batch_size=False,
               loader_prefetch_size=8,
               device_prefetch_size=4):

The fixed_batch_size can fix many problems especially caused by especially transformer type model. On the other hand, prefetch can greatly improve the dataloading speed.
Pitch
I would like to add these to the init parameters of trainers. However, I would like to know whether merge is possible before continue.