docstr: Extract offset part of an indexing expression
code: def offset_var(self, index: Expr, vars: List[sympy.Symbol]) -> Expr:
      """Extract offset part of an indexing expression"""
      index = self.simplify(index)
      return sympy_subs(
          index, {v: sympy.Integer(0) for v in vars if v != 0}
      )
  
imports: import functools
  import itertools
  import logging
  from typing import Any, Callable, Dict, Iterable, List, Optional, Set, Tuple, Union
  import sympy
  from sympy import Expr
  from torch.fx.experimental.symbolic_shapes import ShapeEnv
  from torch.utils._sympy.functions import FloorDiv, ModularIndexing
  from torch.utils._sympy.value_ranges import bound_sympy
  from .utils import sympy_subs, sympy_symbol, VarRanges
  from .virtualized import V
label_desc: Use image node_id 31 for calling the SizeVarAllocator obj's underlying member method code with example usage: obj.offset_var(index, vars) and returns: sympy_subs