Make torch an optional dependency / extra

Modern packages use optional dependecies / extras for specific functionality. I assume this is very well possible for the sagemaker sdk.

A similar request was posted in discussions: https://github.com/aws/sagemaker-python-sdk/discussions/5441

I couldn't find any actual "issue" and hope this gets more visibility.

The main and most urgent topic is the dependency of pytorch which requires 354/620mb size in a clean venv. I would assume that most use cases don't need pytorch at all.

No files or tests are named in the issue. Start by locating the SDK's dependency declarations and installation/extras configuration, then trace where PyTorch is imported or required; compare a clean install with the proposed extra. Done means the base package installs without PyTorch while PyTorch-dependent SageMaker functionality remains usable through an explicit extra.

Written by the indexing model from the issue text.