feat: add convenient selection of data parallelism

[shpface]

Issue #, if available:

Description of changes:
Adds a parameter to easily populate the hyperparameters of CreateJob for the use of sagemaker data parallelism.

Testing done:
unit tests, integ tests

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[ajberdy]

Should we add a note that it's intended for use with >1 instance count?

[shpface]

I don't think so. Data parallel distribution could also be used with a single multi-gpu instance

[ajberdy]

Ah, I didn't realize that. In that case, is there any potential a user may want to create a local job with data parallel distribution if their local hardware supports it?

[yitchen-tim]

I think we can cut local mode out of scope. According to this link, SageMaker local mode does not support distributed training with local GPU

[christianbmadsen]

Let's change the name from distribution to data_parallel

[shpface]

Let's change the name from distribution to data_parallel

The option is kept as distribution : str rather than data_parallel : bool to leave the door open to future distribution methods.