Reference the shared memory object in a worker process when using spawn multiprocessing method

[karan6181]

Description of changes:

• Torch DataLoader uses multiprocessing to spin up num_workers processes.
  • The Linux system by default start method is fork and when a process is forked the child process inherits all the same variables in the same state as they were in the parent.
  • The Mac system by default start method is spawn and when a process is spawned, it begins by starting a new Python interpreter. The current module is reimported and new versions of all the variables are created.
• For the spawn processes, since the resources are not shared, any changes made in next_epoch variable by the num_workers will not be reflected in the main process and this results in the test_streamingdataloader_mid_epoch_resumption test failures when num_workers > 0.
• The changes made in this PR would reference the same shared memory object in the num_workers processes which was created in the main process. This would ensure that the main process or a parent process would see the updated values when changes made by the num_workers processes.

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[dakinggg]

Lgtm