switch to caffe2 without aten

The referenced aten revision is no longer available in the official
repository.  Presumably Caffe2 will be adjusted to work with
the new rewritten repository.
For now, switch to a copy with the aten submodule removed,
since TC does not need a Caffe2 with an aten submodule.

Author: Sven Verdoolaege

.gitmodules

@@ -11,10 +11,12 @@
 	path = third-party/halide
 	url = https://github.com/halide/Halide.git
 
-# Mainstream branches, we don't modify those
+# Temporary hack
 [submodule "third-party/caffe2"]
 	path = third-party/caffe2
-	url = https://github.com/caffe2/caffe2.git
+	url = https://github.com/skimo-openhub/caffe2.git
+
+# Mainstream branches, we don't modify those
 [submodule "third-party/gflags"]
 	path = third-party/gflags
 	url = https://github.com/gflags/gflags