Add docblock to torch/_dynamo/side_effects.py (pytorch#155403)

Add comprehensive module docstring explaining side effect tracking and
management, including mutation tracking, context changes, aliasing,
and state preservation during symbolic execution.

Originally generated by claude but reviewed and edited by me.

Pull Request resolved: pytorch#155403
Approved by: https://github.com/williamwen42

Author: bobrenjc93

torch/_dynamo/side_effects.py

@@ -1,5 +1,28 @@
 # mypy: allow-untyped-defs
 
+"""
+Side effect tracking and management for TorchDynamo's compilation system.
+
+This module provides infrastructure for tracking and managing side effects that occur
+during symbolic execution, including:
+
+- Tracking mutations to objects, attributes, and variables
+- Managing context changes (cell variables, global namespace modifications)
+- Handling aliasing and object identity preservation
+- Managing stack frame state and local variable changes
+- Tracking function calls with side effects
+
+Key classes:
+- SideEffects: Main container for tracking all side effects during execution
+- MutableSideEffects: Specialization for mutable object tracking
+- AttributeMutation/ValueMutation: Track specific types of mutations
+- Various specialized side effect classes for different scenarios
+
+The side effect system ensures that mutations performed during symbolic execution
+are properly replayed during runtime, maintaining the correctness of compiled code
+while enabling optimizations where safe.
+"""
+
 import collections
 import contextlib
 import inspect