[JIT SSA] Allow updating shape functions without recompilation (#83629)

In order to avoid extra round trips, and avoid confusion in places such as
this to manually pull in the latest copy of the shape_functions.py file

This also fixes the cases where people pull in the wrong version of the file. This can happen in cases such as when developers run `python setup.py install` instead of `python setup.py develop` to generate their current copy of Pytorch.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83629
Approved by: https://github.com/davidberard98

torchgen/shape_functions/gen_jit_shape_functions.py

@@ -1,12 +1,34 @@
 #!/usr/bin/env python3
+import importlib.util
 import os
+import sys
 from itertools import chain
 from pathlib import Path
 
-from torch.jit._shape_functions import (
-    bounded_compute_graph_mapping,
-    shape_compute_graph_mapping,
-)
+
+# Manually importing the shape function module based on current directory
+# instead of torch imports to avoid needing to recompile Pytorch before
+# running the script
+
+file_path = Path.cwd() / "torch" / "jit" / "_shape_functions.py"
+module_name = "torch.jit._shape_functions"
+
+err_msg = """Could not find shape functions file, please make sure
+you are in the root directory of the Pytorch git repo"""
+if not file_path.exists():
+    raise Exception(err_msg)
+
+spec = importlib.util.spec_from_file_location(module_name, file_path)
+assert spec is not None
+module = importlib.util.module_from_spec(spec)
+sys.modules[module_name] = module
+assert spec.loader is not None
+assert module is not None
+spec.loader.exec_module(module)
+
+bounded_compute_graph_mapping = module.bounded_compute_graph_mapping
+shape_compute_graph_mapping = module.shape_compute_graph_mapping
+
 
 SHAPE_HEADER = r"""
 /**