Use fault tolerant symlink during training to handle temporary file system inconsistencies.

[mjdenkowski]

Distributed file systems are not always synchronous. For example, a deleted file may still appear to be present until the file system synchronizes. This can cause FileExistsErrors during training when updating symlinks to the best/current parameter files.

This pull request adds and applies a fault tolerant symlink wrapper that makes multiple attempts before raising an OSError. By default, 6 attempts are made over the course of 63 seconds. This should prevent brief file system inconsistencies from causing errors during parameter file symlink updates.

Unrelated: The latest version of deepspeed is not compatible with our current usage. The requirement is updated to deepspeed==0.6.5.

Pull Request Checklist

• Changes are complete (if posting work-in-progress code, prefix your pull request title with '[WIP]'
  until you can check this box.
• Unit tests pass (pytest)
• System tests pass (pytest test/system)
• Passed code style checking (./style-check.sh)
• You have considered writing a test
• Updated major/minor version in sockeye/__init__.py. Major version bump if this is a backwards incompatible change.
• Updated CHANGELOG.md

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.