Disable tracing for Pytorch Mobile client (#36007)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36007

Tracing is not needed in Pytorch Mobile client. Disabling it has a couple of benefits:
1. It's a pre-requisite to build lite interpreter.
2. It saves the code size for full jit and Federated learning (around 600k).

Solution: use PYTORCH_DISABLE_TRACING to disable it. It's better than passing an argument to code-gen because:
1. It's a single-point change in the code template for both VariableType and VariableFactories.
2. code-gen does not handle VariableTypeManual.cpp. The macro is need there anyway.
ghstack-source-id: 101529401

Test Plan: CI

Reviewed By: ljk53

Differential Revision: D20852558

fbshipit-source-id: c28cec9f90208974acfa351ec9aec3fabbbb8aac

tools/setup_helpers/generate_code.py

@@ -25,8 +25,7 @@ def generate_code(ninja_global=None,
                   install_dir=None,
                   subset=None,
                   disable_autograd=False,
-                  selected_op_list_path=None,
-                  disable_trace=False):
+                  selected_op_list_path=None):
     # 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)
@@ -54,7 +53,6 @@ def generate_code(ninja_global=None,
             autograd_gen_dir,
             autograd_dir,
             disable_autograd=disable_autograd,
-            disable_trace=disable_trace,
         )
         gen_jit_dispatch(
             declarations_path or DECLARATIONS_PATH,
@@ -84,12 +82,6 @@ def main():
         '--selected-op-list-path',
         help='Path to the yaml file that contains the list of operators to include for custom build.',
     )
-    parser.add_argument(
-        '--disable_gen_tracing',
-        default=False,
-        action='store_true',
-        help='Disable generating the tracing codes.',
-    )
     options = parser.parse_args()
     generate_code(
         options.ninja_global,
@@ -99,7 +91,6 @@ def main():
         options.subset,
         options.disable_autograd,
         options.selected_op_list_path,
-        options.disable_gen_tracing,
     )