Add warning when importing caffe2 on build without BUILD_CAFFE2=1

Confusing backtraces are issues to user when they try to run tests or actual scripts using Caffe2 on a pytorch build without Caffe2 enabled through BUILD_CAFFE2=1

This PR adds a warning in more than one place to return a friendly message for the user

Pull Request resolved: pytorch#73770
Approved by: https://github.com/BowenBao, https://github.com/malfet, https://github.com/garymm

Author: Thiago Crepaldi

caffe2/__init__.py

@@ -0,0 +1,8 @@
+import warnings
+
+
+try:
+    from caffe2.proto import caffe2_pb2
+except ImportError:
+    warnings.warn("Caffe2 was not built with this PyTorch build. "
+                  "Please enable Caffe2 by building PyTorch from source with `BUILD_CAFFE2=1` flag.")

caffe2/proto/__init__.py

@@ -8,7 +8,13 @@
 #    expected caffe2.NetDef got caffe2.NetDef."
 #
 # This has to be done for all python targets, so listing them here
-from caffe2.proto import caffe2_pb2, metanet_pb2, torch_pb2
+try:
+    from caffe2.proto import caffe2_pb2, metanet_pb2, torch_pb2
+except ImportError as exc:
+    raise ImportError('Caffe2 support is not enabled in this PyTorch build. '
+                      'Please enable Caffe2 by building PyTorch from source with `BUILD_CAFFE2=1` flag.'
+                      '\nDetails from original exception:\n' + str(exc))
+
 try:
     from caffe2.caffe2.fb.session.proto import session_pb2
 except ImportError:

caffe2/python/__init__.py

@@ -1,7 +1,15 @@
-
-from caffe2.proto import caffe2_pb2
 import os
 import sys
+
+
+try:
+    from caffe2.proto import caffe2_pb2
+except ImportError as exc:
+    raise ImportError('Caffe2 support is not enabled in this PyTorch build. '
+                      'Please enable Caffe2 by building PyTorch from source with `BUILD_CAFFE2=1` flag.'
+                      '\nDetails from original exception:\n' + str(exc))
+
+
 # TODO: refactor & remove the following alias
 caffe2_pb2.CPU = caffe2_pb2.PROTO_CPU
 caffe2_pb2.CUDA = caffe2_pb2.PROTO_CUDA