Add docblock to torch/_dynamo/variables/builtin.py (pytorch#155402)

Add comprehensive module docstring explaining built-in function and type
variable tracking, including handling of Python built-ins, type constructors,
operators, and special constructs during symbolic execution.

Originally generated by claude but reviewed and edited by me.

Pull Request resolved: pytorch#155402
Approved by: https://github.com/Skylion007
ghstack dependencies: pytorch#155403

Author: bobrenjc93

torch/_dynamo/variables/builtin.py

@@ -1,5 +1,26 @@
 # mypy: allow-untyped-defs
 
+"""
+Built-in function and type variable tracking for TorchDynamo's symbolic execution.
+
+This module contains variable tracker classes for Python built-in functions, types,
+and operations during graph compilation. It handles symbolic execution of:
+
+- Built-in functions (len, getattr, isinstance, etc.)
+- Type constructors (int, float, str, list, dict, etc.)
+- Built-in operators and methods
+- Special Python constructs (super, hasattr, etc.)
+
+Key classes:
+- BuiltinVariable: Tracks built-in functions and handles their execution
+- TypeVariable: Manages type constructor calls and type checking
+- SuperVariable: Handles super() calls in class hierarchies
+
+These variable trackers ensure that built-in Python operations are correctly
+handled during symbolic execution, either by executing them directly when safe
+or by creating appropriate graph nodes when needed.
+"""
+
 import contextlib
 import functools
 import inspect