incorrect py_version argument documentation

[surajkota]

What did you find confusing? Please describe.
py_version argument documentation is outdated for estimator. e.g.: https://sagemaker.readthedocs.io/en/stable/frameworks/pytorch/sagemaker.pytorch.html?highlight=pytorch#pytorch-estimator
It says py version should be one of ‘py2’ or ‘py3’. Both options are not valid anymore. DLC deprecated py2 and py3 is py38, py39 etc.

Describe how documentation can be improved
Update the documentation to reflect the correct values. I would have opened a PR but I am not aware of a central place where you can see all supported py versions or DLC images. https://github.com/aws/deep-learning-containers/blob/master/available_images.md page only lists the latest few versions

[sage-maker]

#5048