add .gitattributes for EOL conversion. (pytorch#9813)

Summary:
`.bat` file's EOL is LF, so a build is failed on some Windows machines.
To fix this, add `.gitattributes` and set batch file's EOL to CRLF.

Discussion is in pytorch#9677.
Pull Request resolved: pytorch#9813

Differential Revision: D9026486

Pulled By: soumith

fbshipit-source-id: 341eaa677c35f8476a7eda1bac9827385072eb29

Author: shkit

.gitattributes

@@ -0,0 +1 @@
+*.bat	text eol=crlf