Hyperparameters not shell escaped

[bstriner]

Describe the bug
Hyperparameters with spaces get passed as separate command line tokens

To reproduce
Create a hyperparameter like "key" set it to "a b". Toolkit will invoke user script with "--key a b" instead of "--key 'a b'".

Expected behavior
Expect hyperparameters to be passed to script as arguments. Requiring hyperparameters to already be shlex quoted seems unexpected. The commands are correctly passed to Popen but differently parsed when asyncio is used.

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System information
A description of your system.

• Include the version of SageMaker Training Toolkit you are using.
• If you are using a prebuilt Amazon SageMaker Docker image, provide the URL.
• If you are using a custom Docker image, provide:
  • framework name (eg. PyTorch)
  • framework version
  • Python version
  • processing unit type (ie. CPU or GPU)

Additional context
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