torchtext.data.Field deprecated

[jkang1640]

Since the release of v.0.9.0 of torchtext, torchtext.data.Field is deprecated and moved totorchtext.legacy.data.Field(see the list of deprecated classes on their README). This affects most of python scripts in the inputters. For those who use the latest tortchtext, from torchtext.data import FIeld should be from torchtext.legacy.text import Field, otherwise it throws an error. The same goes for the Iterator and Datasets used in the scripts.