Do not build PyTorch with caffe2 by default (#66658)

Summary:
CAFFE2 has been deprecated for a while, but still included in every PyTorch build.
We should stop building it by default, although CI should still validate that caffe2 code is buildable.

Build even fewer dependencies when compiling mobile builds without Caffe2
Introduce `TEST_CAFFE2` in torch.common.utils
Skip `TestQuantizedEmbeddingOps` and `TestJit.test_old_models_bc`  is code is compiled without Caffe2
Should be landed after https://github.com/pytorch/builder/pull/864

Pull Request resolved: https://github.com/pytorch/pytorch/pull/66658

Reviewed By: driazati, seemethere, janeyx99

Differential Revision: D31669156

Pulled By: malfet

fbshipit-source-id: 1cc45e2d402daf913a4685eb9f841cc3863e458d

setup.py

@@ -824,23 +824,24 @@ def configure_extension_build():
 
     # These extensions are built by cmake and copied manually in build_extensions()
     # inside the build_ext implementation
-    extensions.append(
-        Extension(
-            name=str('caffe2.python.caffe2_pybind11_state'),
-            sources=[]),
-    )
-    if cmake_cache_vars['USE_CUDA']:
+    if cmake_cache_vars['BUILD_CAFFE2']:
         extensions.append(
             Extension(
-                name=str('caffe2.python.caffe2_pybind11_state_gpu'),
-                sources=[]),
-        )
-    if cmake_cache_vars['USE_ROCM']:
-        extensions.append(
-            Extension(
-                name=str('caffe2.python.caffe2_pybind11_state_hip'),
+                name=str('caffe2.python.caffe2_pybind11_state'),
                 sources=[]),
         )
+        if cmake_cache_vars['USE_CUDA']:
+            extensions.append(
+                Extension(
+                    name=str('caffe2.python.caffe2_pybind11_state_gpu'),
+                    sources=[]),
+            )
+        if cmake_cache_vars['USE_ROCM']:
+            extensions.append(
+                Extension(
+                    name=str('caffe2.python.caffe2_pybind11_state_hip'),
+                    sources=[]),
+            )
 
     cmdclass = {
         'bdist_wheel': wheel_concatenate,