[WeAreResilience]

Issue #, if available: #6459

Description of changes:

_create_sagemaker_model creates the model through self.sagemaker_session, then reloads it with Model.get(model_name=..., region=...). With no session, the describe goes through the default boto session, so if that session points at a different account the call fails because the model only exists in the target account.

The fix passes session=self.sagemaker_session.boto_session to Model.get, the same way Transformer already does in sagemaker-core. The PipelineSession early return is unchanged.

Added tests/unit/test_model_builder_model_get_session.py, which checks that Model.get receives the configured boto session. It fails on master and passes with the fix.

Testing: in sagemaker-serve, tox -e flake8,black-check,docstyle,pylint passes, and so does the new test. The full tests/unit run gave 2076 passed and 65 failed, and the same 65 tests also fail on master in my environment, so the fix doesn't add any failures. I didn't reproduce this against two real AWS accounts.

_get_model_from_endpoint and _find_reusable_model call Model.get without the session as well. I kept them out of this change to stay within the issue, but I can follow up on them if you'd like.

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[WeAreResilience]

Thanks @mohamedzeidan2021, #6487 covers this and also handles the other Model reloads, so I'm closing this one in favor of it. Glad to see #6459 fixed.