code: def __enter__(self):
      res = super().__enter__()
      if self.mmap is None:
          self.mmap = np.memmap(
              self.manager.reader.file_name, "uint8", mode="r"
          )
      return res
  
file_path: ffcv/ffcv/memory_managers/os_cache.py
imports: from typing import TYPE_CHECKING
  import numpy
  import numba
  from .base import MemoryManager, MemoryContext
  from ..pipeline.compiler import Compiler
label_desc: Use image node_id 3 for calling the OSCacheContext obj's underlying member method code with example usage: obj.__enter__() and returns: res