introduce INTERN_DISABLE_AUTOGRAD flag to create inference only library for mobile

Summary:
This is the first of a series of changes to reduce build size by cutting
autograd functions from mobile build.

When INTERN_DISABLE_AUTOGRAD is set:
* On CMake side we exclude Functions.h/cpp, VariableType*.h/cpp,
  VariableTypeManual.cpp from the build process. Still keep variable_factories.h
  as we rely on it to create variables instead of tensors.
* In source code we gate a couple autograd references (in autograd/variable.cpp)
  with C10_MOBILE (technically we should use a dedicated c macro but its
  maintenance cost is higher than cmake macro as we have several build systems
  to change).
* Pass --disable-autograd flag to codegen script, which will stop generating
  Functions/VariableType code. And for variable_factories.h it will stop
  generating tracing code.

Edit: in this diff we will keep Functions.h/cpp to avoid changing source code.

Why we need this change if it's already not calling VariableType and autograd
stuff with USE_STATIC_DISPATCH=ON for mobile?
It's trying to reduce static library size for iOS build, for which it's
relatively harder to strip size with linker approach.

Why we need make involved change into codegen script?
There isn't a global config system in codegen - autograd/env.py provides similar
functionality but it says not adding anything there.

Test Plan:
- will check CI;
- test mobile build in sample app;

Differential Revision: D17202733

Pulled By: ljk53

fbshipit-source-id: 5701c6639b39ce58aba9bf5489a08d30d1dcd299

tools/setup_helpers/generate_code.py

@@ -23,7 +23,8 @@ def generate_code(ninja_global=None,
                   declarations_path=None,
                   nn_path=None,
                   install_dir=None,
-                  subset=None):
+                  subset=None,
+                  disable_autograd=False):
     # cwrap depends on pyyaml, so we can't import it earlier
     root = os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
     sys.path.insert(0, root)
@@ -41,7 +42,12 @@ def generate_code(ninja_global=None,
         gen_autograd_python(declarations_path or DECLARATIONS_PATH, autograd_gen_dir, 'tools/autograd')
 
     if subset == "libtorch" or not subset:
-        gen_autograd(declarations_path or DECLARATIONS_PATH, autograd_gen_dir, 'tools/autograd')
+        gen_autograd(
+            declarations_path or DECLARATIONS_PATH,
+            autograd_gen_dir,
+            'tools/autograd',
+            disable_autograd=disable_autograd,
+        )
         gen_jit_dispatch(declarations_path or DECLARATIONS_PATH, jit_gen_dir, 'tools/jit/templates')
 
 
@@ -55,6 +61,12 @@ def main():
         '--subset',
         help='Subset of source files to generate. Can be "libtorch" or "pybindings". Generates both when omitted.'
     )
+    parser.add_argument(
+        '--disable-autograd',
+        default=False,
+        action='store_true',
+        help='It can skip generating autograd related code when the flag is set',
+    )
     options = parser.parse_args()
     generate_code(
         options.ninja_global,
@@ -62,6 +74,7 @@ def main():
         options.nn_path,
         options.install_dir,
         options.subset,
+        options.disable_autograd,
     )