move PATH_MANAGER to OSS

Summary:
## Problem:
pytext got "No module named 'pytorch'" in issue facebookresearch/pytext#1706

It's due to `from pytorch.text.fb.utils import PATH_MANAGER` is internal only but imported in pytext. Actually, `pytorch/text/fb/utils/__init__.py` should be open sourced.

## Solution:
This diff moved it to OSS as `from torchtext.utils import PATH_MANAGER` and updated all the references

Reviewed By: Nayef211

Differential Revision: D39292896

fbshipit-source-id: c0046d62e64145b60ad9a5298b366f0f1a348369

torchtext/utils.py

@@ -6,6 +6,7 @@
 import zipfile
 
 import torch
+from iopath.common.file_io import PathManager
 from torchtext import _CACHE_DIR
 
 from ._download_hooks import _DATASET_DOWNLOAD_MANAGER
@@ -228,3 +229,16 @@ def get_asset_local_path(asset_path: str, overwite=False) -> str:
  else:
  local_path = download_from_url(url=asset_path, root=_CACHE_DIR, overwrite=overwite)
  return local_path
+
+
+PATH_MANAGER = PathManager()
+'''
+We use iopath to handle local files, remote files with http/https urls, etc.
+This global instance is registered with all the required handlers with the best
+defaults. Learn more about iopath: https://github.com/facebookresearch/iopath
+
+Examples:
+ >>> from torchtext.utils import PATH_MANAGER
+ >>> with PATH_MANAGER.open(FILE_PATH) as f:
+ >>> f.read()
+'''