move PATH_MANAGER to OSS

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

pytext/utils/file_io.py

@@ -9,7 +9,7 @@
 # the import everywhere in PyText
 # TODO: @stevenliu use PathManagerFactory after it's released to PyPI
 from iopath.common.file_io import HTTPURLHandler
-from pytorch.text.fb.utils import PATH_MANAGER as PathManager  # noqa
+from torchtext.utils import PATH_MANAGER as PathManager  # noqa
 
 
 def register_http_url_handler():